I don't think that love exists for a single specific reason - it means many different things to different people.
For religious people, God and love are very tied together. For others love is a purely scientific product, and to many it's the aspect of life that brings them the most joy and a reason to live.
Personally, I think that love (as an emotion) can be the most difficult and frustrating to deal with, but provides the greatest rewards. It's the epitome of sharing life with another person, which for many is what living is all about.