In general, or with particular people?
As social creatures, humans instinctually seek connections with others. Some would say that our reproductive urges drive us to pair off, but that doesn't explain why we fall in love after our prime mating years are over, or with same-sex partners - and that surely does happen! We're certainly socialized to pair off, even at very early ages, and the romantic ideal of being "in love" is a strong theme in western culture.
As to why we fall in love with one person in particular, that has a great deal to do with the person doing the falling, the person for whom he or she is falling, and the situation. There's a word, "limerence," that describes the feelings one experiences when "in love" better than the awkward phrase "being in love" really does, but unfortunately few people are very familiar with it.
In any case, when we fall in love we are usually in a situation in which we are wanting to connect with someone. The person on whom we fixate may or may not be particularly appropriate for a long-term relationship, but limerence cares not a whit. The tremendous highs and lows of emotion can be addictive, and if they are mutual the situation is one with the potential for incredible pleasure, dramatic tension, and sudden crashes into despair. It may or may not lead to an actual "relationship" (meaning, to my way of thinking, something lasting more than a year), but it is what the western world has trained most of us to pine for. Twoo Wuv!
The chemicals released during limerence are so addictive that some people aren't happy unless they are in that stage, so they never actually move beyond it into a deeper relationship. They just go from one new "soulmate" to the next. (Of course, they are often the same people who aren't really willing to do the real work of building and sustaining a serious relationship, or perhaps they don't even understand what is necessary, but that's another post for another time.)
I hope this came close to answering your question in some respect. If it didn't, please let me know and I'll happily make another attempt.
Namaste,
Cyn