Why do people call me selfish?

My family calls me selfish sometimes. They said I dont give back enough. My stepmom said not even once I gave my friends gas money and all and that is probably why they refuse to offering me a ride. This little child yesterday asked to make a donation and I yelled and said, "What do you think I am, your fairy god mother?! Beat it!" She cried and ran off. Her mouth confronted me about what a selfish monster I am. Even my best friend was disappointed. She was like, "What the hell was that?! That is NOT my best friend because she doesn't treat younger children like that."
I'm usually selfish, especially with my money and things. Truthfully I rather receive but I sill give. Now I feel bad.