Humans evolved to be social creatures. Our brains are wired for love, as are many social animals. Dogs didn't take treats from strangers who mistreated their owners in studies done. By loving a close kit group and sacrificing for them, your chances of survival raise heavily. Love is a great survival tool. Casual sex doesn't work because sex releases endorphins and hormones of intense emotion. People who try and ignore it or go through that intense bonding with multiple people end up as damaged goods.