I don't really get the question - I mean I understand it just fine. But on another level, I just don't get what is so wrong about being selfish. What I think people mean when they say someone is selfish is that the person is not doing something they want them to do, is not meeting their needs. Sometimes it is not possible or healthy to meet another's needs. I think it is healthy to put oneself first sometimes.
I think having a strong sense of self is essential to being a healthy person. Knowing who you are includes knowing what you want, what you can give to others and so on. If you know what you need to have your personal needs met, then that is a positive thing because you can then advocate for yourself with others - family, friends, lovers, partners, etc.
I've noticed that people who worry a lot about being considered selfish often struggle with boundaries - both setting one's own boundaries and respecting the boundaries of others. If you don't feel like you have the right to set a boundary for yourself, or don't expect others to respect your boundaries, then it can be hard to have a strong sense of self, to know where 'you' end and others begin. I consider boundary setting and respecting (as well as negotiating boundaries) to be a critical adult skill.
@choctaw103, I laughed when I read your post. I'm an only child too. I have to actively remember 'it's not all about me' when dealing with other folks.