Why is one selfish? Selfishness arises from the feeling of incompleteness or insecurity. We are not content with what we have and thereby want more of it believing it would make us feel better. A bar of chocolate when broken into two would usually undergo the scrutiny as to the size and length. Among mature friends it would go with a smirk and among juveniles it could be a fight.
Whatever a human does he does it for his need. Without anything for him in it he wouldn't lift a finger to make that great sacrifice. The reward could be money, pleasure, recognition, self esteem - anything that boosts his ego. A casual meeting of friends is to have fun. A couple deciding to have a kid is for their need maybe to have a lineage in place. Some do charity to derive happiness out of giving. They can't give away things that are of utmost important to them but only part those they can be without. Most parents sacrifice to see their kids do well because their happiness is dependent on how well the kids fare in life. At the surface it would look selfless but if the acts of sacrifice or charity give them displeasure they wouldn't continue. This is how existence is. There is nothing wrong about it.
The same person could be selfish in some acts and may appear not so in some other acts. In some aspects of life they feel incomplete while content with some other aspects. They could be content having fed well that they may not fight for a meal but could show their inadequate self in a traffic jam by tailing another vehicle blocking the pedestrian space to cross. It is so hard wired that they do it so subtly so well without awareness.
When the self looks at itself it mostly sees its inadequacy. It sees what is not. It is this inadequate feeling that leads to sadness and acts of selfishness. From the way one fills his meal plate to the speed and way one eats that food. From the way people talk to the way people dress up would exhibit their inadequacy. It is just the level and degree of inadequacy that varies. Some are called as selfish while the subtler ones could end up as philanthropists.
Selfishness would disappear when the self sees its abundant nature. When it realizes it is already complete and that ignorance of its abundance brought about the absurd behavior. When selfishness drops you would be a compassionate being.