All living things are selfish, because it is behavior that increases fitness (measured as the number of offspring one had that survives to reproduce). Altruism is so rare (because it decreases fitness) that many scientists mistakenly call a species altruistic if they are not overtly selfish even if it costs them nothing. For example, if a person sees someone drop a $20 bill and returns it to the owner, then some scientists would label the good Samaitan's behavior "altruistic."
Humans evolved to be social animals, because we need others to survive in the dangerous African savanna. In order to survive and live together in harmony, humans must refrain from being too selfish. That is indeed what good parents do, they make sure that their children share. Wolves are also social animals, and they live in packs. In a video documentary the alpha wolf's daughter tried to be selfish and monopolized a piece of food instead of sharing it, the alpha male punished her by biting her and made her relinguish control. Therefore it would appear that animals that are social must play fair and share. Therefore all social animals, not just humans, must find a balance between sharing and being selfish. Indeed, that is why many people steal, because taking other people's property is anti-social behavior and it will result in punishment by society. Stealing will often allow people to commit a selfish act without being detected and punished. Stealing therefore is the result of hypocrisy. People can pretend to be better than they really are by stealing instead of robbing. Animals steal too. A monkey for example was seen hiding a piece of food by dropping it into the water instead of eating it right away since it would get punished for not sharing. Only after the other monkeys within the social group have left the area did the monkey retrieve the food and eat it by itself.