While I don’t doubt that atheists can love and that many love deeply, their materialistic world view prevents them from having a rational basis for loving in every instance.
To see why, let me start by proposing a working definition of love, one that pertains specifically to loving others and one that might be satisfactory to rational people of all faiths or even none. I propose: “Love is freely choosing another’s good.”
This definition, originally from Thomas Aquinas, is simple, acceptable to people from many backgrounds and perspectives, and very broad and inclusive. It does not exclude feelings and emotions like romance and sexual attraction, but it does not demand them either. The good it references could be the mutual pleasure of two lovers sharing a candlelit dinner together, or the care a man gives to his disabled wife. It might involve a small act of thoughtfulness or a heroic act of self-sacrifice. Note too, that it states that the choice to love should be free, not forced.
In this definition, “love” is intrinsically tied to another’s “good”, but what is “good”? To an atheist, all reality is material and nothing spiritual exists that transcends the material and physical. So any notion of “good” and all motives for choosing good must be based upon purely materialistic principles. Most modern atheists attribute human altruistic tendencies and love to evolution. That is, love is an evolutionary impulse that serves to promote the thriving of human life on earth. The working definition of love that I proposed above does not exclude the possibility that evolution has contributed to the human drive to love, but there is nothing in a purely material perspective that would mandate that the “survival of the fittest” be considered the paramount good, over and above any individual good. One reason is that for a materialist, death ends the possibility of the enjoyment of any good for an individual, so how could the choice of a good that could not be realized be a rational choice for that individual?
Now consider the case of caring for a permanently and seriously disabled person, such as the example of a man caring for his seriously and permanently disabled wife. Suppose the woman is bedridden, unable to meet most of her needs on her own and the man has no hope of having an intimate emotional and/or sexual relationship with her ever again. While evolution might explain the man’s caring instinct in general, a strictly materialistic perspective would not provide any basis whatsoever for the man expending his time, energy and comfort. i.e. his own good, for caring for his wife.
Loving often has a cost, and the cost is some loss of one’s own good. Throughout all cultures and time we see that people measure love by it’s cost and that generosity is most often seen as the measure of love. Atheistic materialism fails to provide a rational basis for loving when it would result in a net loss of one’s own temporal good. For example, if it were otherwise, then the world would probably be seeing more orphanages and the like being built by avowed atheists. Atheists, if they wish to only love rationally, have only two choices: either refrain from loving at the cost of their own good, or admit that some good exists that transcends physical reality and is accessible to those who love at a cost to themselves.