Without love and bonding there would be no reason for parents to raise their children, greatly diminishing their chances for survival since human babies are essentially defenseless and weak creatures. Even with these chemical reactions going on in the brain causing the feelings of love, some parents STILL abandon their children. Consequently, Love is also what builds a family, a community. We can see what people without Love are like. Sociopaths, serial killers. Think about the survival time of a serial killer in the wild. He has no friends, no one to help him and is constantly looking for battle/combat just to feel alive.
The only answer there can be in regards to your question and evolution is that it increases survival. It's really not anymore complex than that.