Wow...where to start...
The term "love" is ambiguous...everyone who tries to define it comes up with a different definition and a lot of ppl refuse to even try...
There is definitely a brain chemical response/shift that has physical and emotional effects, when ppl feel an "attraction" to others. This plays a large part in the continuation of our species. And not only because it promotes reproductive functions, but also because it causes humans to be protective about the people they "love", their families, whether by blood or bond (i.e. spouses, partners, etc.). Without this protective instinct that causes grouping, humans would not have survived.
But I think that now, when basic necessities of survival are in the back of the minds of those of us who live in industrialized countries, we're searching for a deeper "connection". We are searching for someone who can "understand" us and who appreciates our personalities. Using the Yin-Yang as a representation, I think we are looking for someone whose personality is the best compliment to ours, that is, it fits with ours. If the pieces don't fit well together, there will be poking and gaps and tension that will eventually lead to separation. I think we are searching for our compliments, and when we find them, we will want to be together and stay together. To use that phrase from Jerry McGuire...we are searching for the person who best "completes" us...