That is a good question on why people are selfish. According, to me this is selfishness builds up due to jealousy and greed for more money I would say. If, you look at two person, say for example a person who is happy and is contemned will not at all be selfish and will be a kind of person who is willing to support. Whereas if other person who is jealousy with others success and is a greedy person you cannot expect good deeds from this person. Altogether it drills down to people upbringing and how their are behaving in the realistic world.