For me, there are a lot of reasons why a person is having that behavior (selfishness) and his experience plays a big part on it. These my hypothesis on why people become selfish in today's world.

- Environment. 

They lived in a community(place) where the nature is being independent in yourself. That you don't need to beg and live in your own.

- Experience.

They had experienced mess and testing that made them realized they need to be individualistic - removing the idea of sharing in their minds.

- It is enough for them.

Sometimes we feel their selfishness but we do not understand that the things that they have is enough for them. Sometimes they feel they worked hard for it and deserves their reward.