Selflessness requires a self
Check this out. Bacteria, microorganisms or whatever - have no sense of self. they are ultimately selfish, eating everything within reach. A fish is going to eat the worm. A cute little critter is going to happily eat the head off a bug, and do it competitively with other little critters.
A plant does not have a way to consider giving other plants below them some light - because they have no concept of self to set aside - they need a self to be selfless.
So things without self, are self-ish, and things with a self are empowered to be self-less.