Evolutionarily, we love to continue the species. If you mean the lust and procreate kind of love. If you are referring to why we stay with people and take care of them, there's an evolutionary answer there too. A child was more likely to live if both parents were there to take care of it, protect it and provide for it.
I believe that love is a natural emotion and feeling just as fear, anger, and other emotions are. These are not emotions that anyone has to learn to do, they just come naturally. Perhaps they are all a result of survival as a species and human interaction.
Man is by laws of nature a social animal. Man is also the only mammal that uses sexual intercourse for reasons other than procreation. Both these would provide logical and natural reasons to experience love. That being said, perhaps we love others who bring balance to ourselves. Not those who complete us, because we cannot be completed by others. But others whom we love can bring us balance.
There is no way to know why people love each other. There are so many different possibilities. For example:
- A religious person could argue that people love each other because they are imitating God or doing the will of God. They could say that God loves people and so people love each other just as God tells them to.
- A romantic could simply say that it is in our nature to love because love is fulfilling. Such a person could say that when we love, we are truly happy. Therefore, we love each other so as to make ourselves happy.
- A person who believes more in physical phenomena might argue that love is an illusion. They might say that we make ourselves think we love. They might say that we act as if we love people because it helps us procreate and helps our descendants survive. By this way of thinking, "love" is an evolutionary adaptation.
But there is no way to know for sure.
They don't do it...
And how is this anywhere near relevant to anything at all?