What is selfishness?

I kinda struggle with the concept a bit. I mean, if everyone were to be selfless (i.e. putting what other people want before what we want) like we are theoretically supposed to be we'd end up on this merry-go-round of selflessness, never getting anything done-

A: "So what do you want to do, B?"
B: "Well, actually, what do YOU want to do A? I really don't mind."
A: "No, really B, I insist! What do you want to do?"

And so forth until finally someone is forced to make their wishes known, hence forced into a "selfish" position.

What about this one- if A wants to do something with B who knows of this desire, but is busy, who is more selfish? A because he wants to spend time with B in the first place? Or B for not making time for A? I've tried to get my head around that one for YONKERS but can't find a resolution. Maybe my logic and general understanding is all whacked out. I dunno! What think you, lovely readers?