If you are a Believer, then you know why it exists. However, we will take it that you are not and just go with the garden variety secular slant. Even if one believes the world is just the world, certain things are hardwired in most of us, that is to get with someone special of the opposite sex. Bypassing some intelligent design that placed the desire there, you have to go with nature, any species that was not hermaphrodite would be extinct pretty quickly if that was not the usual order of things. That is the Cliff Notes of it but it goes to more than that. The other junk, jealousy, coveting, bitterness, etc. that accompany relationships are because of immaturity of the participants or just their nature because they have nothing more to aspire to. ,