Why is it hard to say No?

I have always had troubles saying no to anyone. If I say no then I feel bad for along time then I go ahead and let them do it.  Where I work I never say no to anyone if they ask me to do this or that. Why is that?  I don't know I guess I don't like to make people upset so I say yes even if its more work for me.  Or if I was going to get something for me and they wanted this or that and something had to go it would be mine. I was sitting today at working in between lunch and going back and asking myself why do I always do this to myself, why do I say yes to everything and wear myself down. Why can't I just say no and not feel bad and know its ok to say no sometime. Everyone has the right to say no, it doesn't mean you are a bad person it just means you can't do it at this time but next time you might be able too.  I need to get stronger and know its ok to say no and when you say no doesn't mean you are bad or anything just means that sometime you can't do or you don't have the time.