Some people maintain that kindness is a function of the heart, and that rationality can say little about it, and others maintain that being kind is a social or religious mandate. In both cases, however, we may talk about the feelings, or talk about the mandate.
The science of kindness is not opposed to religious or mystical traditions. In keeping with the apophatic spiritual traditions, we may even conclude that kindness is just inherent, and not need to justify why one should be kind. In that case, however, it is important to examine any mistaken reasoning behind the science of selfishness.
Finally, why use science, and why study kindness? We use science because even though it has a number of unexamined assumptions, such as faith in randomness, the scientific method still represents one of the best tools available to determine intersubjective truth. We study kindness because it increases the happiness of both others and ourselves.