The first person I ever 'fell in love with' was my next-door neighbour and he was 7 and I was 17ish. I remember rushing to a priest friend (a monk actually) once the agony of my growing love for this little boy became too much and how his response changed my life. I don't remember his exact words but he basically told me what wonderful news it was! (He was such a special man.)
So perhaps it is only when we love another person - however tragic, hopeless and ridiculous that love may seem to be - that we can even begin to understand how to love God. I can see now - many years later - how this relationship - doomed though it was in one way - taught me more about God than any amount of studying books could have done.
So my answer to your question 'why people love' has to be that that is why we are here! And it truly is God' greatest gift to us but I think that we can only understand this through prayer (and yeah quite a bit of pain too come to think of it)