Thoughts on A Raisin in the Sun: Selfishness

There is no doubt that Walter acts selfishly in the novel -- he takes Mama's money and spends it on his own dream, and he ignores his family's advice/directions, even though what he does directly affects them. He loses the money that has the potential to take them out of poverty and fulfill part of their collective dream; that is the lowest point of the play, and in that moment it seems that all hope is lost.

So why was Walter such an appealing character to me? As I read the book, I found myself siding with him, becoming angry with Beneatha for treating him so coldly, and excusing his drinking and irresponsibility.

It all goes back to the first scene, in which Walter passionately defends his dream, his desire to take control of his situation and provide for his family. I forgave him because his selfish actions stem from unselfish desires. I identified with his passion, his longing for a single goal, and accordingly overlooked his foolish and selfish actions.

One major point in Walter's personality is that he's often willing to risk too much or give too much of himself to attain society's definition of success. In the end, though, I can't condemn him for wanting this kind of success, for I am often guilty of making these same types of sacrifices.

What exactly defines selfishness? 
Can an action ever be wholly selfish or unselfish? 
And to what extent is selfishness acceptable?