When I think of the word (selfish) I think in terms of someone who is unwilling to "share". They want it all to themselves.
I don't believe it's possible to be selfish "full-time". Most "healthy" people want to make friends and eventually find love or gain praise of some kind from others. What normally happens is those who are called "selfish" most likely are just very "selective" with whom they (share) with. One man's opinion! :-)
There are only two ways to experience joy and happiness in life. We either get what we want or we learn to be happy with what we have. Be careful when others label you various names. Most likely it's a manipulation ploy to get you to do what THEY want you to do. Life is a personal journey! You have to run your own race.