Just to play devil's advocate here, most people actually do enjoy helping others. If you're the rare person that doesn't like helping other people at all, then it's probably a waste of my effort to try to make you. So "do what you like to do" != "be selfish" in all cases.
That's basically what it is, and maybe also a mindset of believing you matter as your own person, rather than just a means to and ends. Having at least some selfishness of that sort (at least when living in a culture designed for it - maybe things are different in more group-oriented cultures) is really important for things like being confident, being able to say no to others' requests, and not getting depressed.
Why wouldn't you advocate pure selfishness? Selfishness is a Good Thing. Why do we do anything? Because it satisfies some internal desire or drive. Selfishness is the epitome of living.
How does what you're advocating differ from pure selfishness (perhaps restricted by the requirements to not to hurt others)?