This is not a small question. Honestly, there is probably more we don't know about the neurology and physiology of love than we currently do, but...I'll give it a go.
First off, let's clarify what we mean, here. From the way the question was phrased, I'm pretty sure we're talking about romantic love, since the word attracted was used. That said, it is worth noting that there is a sliver of overlap in the Venn diagram of romantic love and "other love." At the very least, there is a common element of obsessiveness that can be found in a romantic crush, in a parent's gaze at their newborn, and in some cases when someone plays with their new puppy or kitten. The love interest becomes a consistent object of thought.
Chemically, there are a few things going on here. The biggies are dopamine, serotonin, and oxytocin. And vasopressin. And others. I'm going to avoid the details on these, other than to just say that when it comes to love, there are a lot of things that go on in your brain. Some are associated with the feeling of excitement and satisfaction. Some simply make you feel good. Some are associated with bonding. Instead of trying to address the question of whether these are causes or effects, and what role each exactly plays, it might be simpler to profile the overall process of romantic love.
Stage 1: You go crazy. You fixate and obsess on your love. When you spend time with them, you are filled with good feelings, a dizzy euphoria. When you are away from them, it is an agony.
Stage 2: You sober up. They seem pretty good, maybe. Maybe you'll stick around. Maybe not. If you do...
Stage 3a: You build affection. If you've stuck around, it is possible to grow to love your partner in a different way, with a stable and supportive relationship. You may no longer get dizzy when around them. Sparks no longer leap between your fingers, but....you miss them strongly when they're gone. They make your life better. You're still attracted to them, but not as passionately.
Stage 3b: You go crazy for someone new, usually leaving your old love behind.
So, why?
Well, there are a number of factors as work. Some are obvious, some less so. Let's start with some of the obvious cultural and social elements.
As advanced as we like to think we are, as pure as our intentions and minds may be, we do not operate or love in a vacuum. First off, attraction is partially based on physical appearance, which has a huge cultural element. What we as individuals consider to be handsome or beautiful typically reflects what our society considers to be beautiful also...and society tends to find success beautiful. In places where access to food reflects economic (and therefore probably reproductive) success, a heavy body weight can be sexy. In others, access to specific materials such as clothing or technology can similarly be interpreted as attractive. In the USA, we tend to idolize people who have the money and resources to pursue a youthful appearance...or just those who are youthful and healthy.
Most research shows, though, that people tend to form stable relationships with others of similar attractiveness. "Marrying up" is rare because "marrying down" is rare. If people can trade up for a more attractive partner, they normally do.
So, there is that basic equation: the evaluation where some part of our brains ask if that person we're looking at is both attractive according to our culture and approachable to ourselves. It should be noted that when it comes to romantic love, many of the "decisions" we make (consciously or unconsciously) reflect the basic goal that mature humans have of making babies. While it may be cute when two 5 year olds introduce each other as boyfriend and girlfriend, and announce their plans to marry, no one actually believes that they are in love. The insanity of genuine romance requires a body that is either going through puberty or has already gone through it. The development of sexual characteristics (sorry) is a prerequisite for the hormonal elements of romantic love. Therefore, a lot of what we find attractive revolves around the characteristics necessary to make healthy babies, or to take care of them once they are born. I'm not going to go into that too much, because...awkward!
So, out of the pool of people that are the right level of attractiveness, why do some cause us to "fall in love?" Well, a plethora of research shows that a lot of it has to do with diversity and smell.
The smell part is the role of pheromones. Humans are animals, and like most animals, we communicate information through the use of chemicals that we secrete. Some of these can move through the air. When we smell another human, we are typically not aware of it. The effect is small, but significant. For example, in a classic experiment, a group of men and women were given a set of photos to grade for attractiveness level. People consistently increased the attractiveness ratings for photographs they looked at while also exposed to human pheromones. Basically, pheromones have a role in causing attraction. Since these chemical signals are based on genes, it also makes sense that we would be less attracted to our own and those of close family members. Conversely, then, we should be more attracted to people who smell differently than our families, and this is exactly what the research shows. That's the diversity bit. This also shows up when it comes to physical appearance. When most of us envision the opposite sex as essentially ourself with different hair and clothing, I doubt many of us would express any attraction. That person would look like a sibling, not a possible mate. So, we also tend to value some diversity in appearance.
Interestingly, there have also been some studies that show that relationships that begin when one or both partners are on medications that alter their hormonal and pheremonal signatures (the smells typical of that person,) don't last as long. This is presumably because when the drugs are stopped, the person's smell changes, and they no longer seem as attractive to their partner.
So, one alternative for why people fall in love is...they smell right. This is a good explanation, especially because it correlates highly with stable long term relationships. (There are some decent evolutionary arguments why humans would turn out to find mates based on smells that show they are sufficiently different. It increases variability in offspring, and reduces the likelihood of negative recessive traits showing up.)
The other alternatives are either a malfunctioning system, or an incomplete system. What do I mean by that? Well, it is obvious that in some cases, something goes awry. People can become obsessed and attracted to people they've never met, or to people who are completely inappropriate as romantic partners for any of a number of reasons. This could be a result of unusual brain chemistry or environment or both.
As for an incomplete system...this is the result of insufficient information, either about the self, or the potential partner. You either don't know what you are really attracted to yet, due to youth or inexperience. This can lead to a lot of serial crushes and playing the field. Similarly, it is entirely possible that basic pheromones will tell you you're attracted to someone, but then you find out you can't stand to watch them chew food. In either case, the romance as such is a result of bad information, and will typically end.
Okay, now all that is said,there is finally what I personally consider to be the most important element: opportunism. IF love and attraction only depended on the right chemistry, then love would be rare. But it isn't. So either most pheremone combinations are good combinations, or the environment can make a huge difference. And it can. The idea of romantic love is a relatively modern one. Historically, most marriages were arranged, and still are in many parts of the world. But those marriages still can lead to romantic love. So...why? Well, probably for the same reason that two people on a deserted isle will find a way to make it work. Love is opportunistic. Without choices, on some level we can choose to love.
There are many who may feel that this is insufficiently romantic. The idea of a soul-mate is a powerful one. but it may also be unrealistic, and lead to a lifetime of searching. For my part, I have been happily in all these kinds of love for decades, now. And if there is an element of choosing to actively continue that love, rather than just hope it continues, then I am comfortable with that.