"No"...the nice girls four letter word

Ive seen the error in my unselfish ways!!! Nice people get taken advantage of and disrespected. People get mad when nice people say no! If I express the fact that i don't want to do something or anything, i am expected to give an explanation.......as if no, requires an explanation! If i don't then im in a bad mood. What! If people think im in a bad mood they keep away from me. No "whats wrong?" or "R you ok?" uh uh no. Just shunned. But what kills me is .....im not in a bad mood, im just saying no. Geez!