Loving behavior contributes to the group at the expense of the individual. Competition contributes to the survival of the individual at the expense of the group. In the garden of life, some people are more like flowers, and other people are more like weeds.
The magnitude of an action may change not only the strength of its impact, but the direction. If you became a dentist, for example, you would certainly be an asset to our society. But what if everyone became a dentist? Who would bake the bread? Who would build the houses?
It is a lot easier to prove that you don't love someone than it is to prove that you do, but one of the best 'proofs' I know is the desire to devote time to the person with no expectation of any sort of compensation, including gratitude, in return.