mean vs. nice

Why is it so much easier to be mean. I did a really good deed for my mom this week. She has praised me nonstop for going the distance for her....I finally asked her "Was I so bad before?". I know I am not the nicest person in the world and I am well known for my verbal diarrhea but I didn't think I was that bad. My mother and I have always had kind of a strained relationship. It has only been recent that we are good friends. I love and respect her but I don't like her very much sometimes. I think she can be selfish and self-centered and I think she has the same opinion of me. I have four brothers and sisters and I am definitely NOT the squeeky wheel. I'm painfully independent. I could count on one hand the times I have asked her for help. I have always paid back money when I owed it. I always help with holiday preparation. I say this because my brothers and sister DO NOT do these things. I just keep thinking about what my mom always says..."we don't do nice things for the reward we might receive for doing them".