To have or not to have a baby, THAT is the question

Some people choose to go through life without reproducing, without having a baby, and are called selfish for that by others.
IS it selfishness that causes people to decide against having kids?

My education is in Biology, so allow me to put my answer in biological terms: Adaptation happens when an organism evolves to best interact with its environment; humans change their environment rather than adapt to it, so natural selection is no longer based on environmental factors such as temperature, humidity or local predators. Consequently, some of our "natural" drives are no longer needed, or even desired.
There are enough babies in the world right now, too many in fact; to say that those who choose not to have children are selfish is rather insipid. The selfish choice, to me, seems to be the indulgence of the biological drive to reproduce in spite of growing evidence that our rate of growth is killing the planet.
I don't know if I'll have children or not, but I have gratitude and respect to those who choose not to.