Why is it so hard to say no to someone?

Every day we constantly have people asking us to do things for them that the majority of the time we wish they would just do themselves. However, we always seem to consent to do the task anyway even though we wish that we could be doing something that we enjoy. The thing is, it's not just one assignment at a time; it's a variety of different people asking you to do different things. At some points it is so frusterating and you just want to say no to the next person who talks to you, but when you are asked to do something you accept the offer so happily. It is confusing and sometimes nothing but a huge joke in our lives. If we could acctually learn to say no once in a while, maybe just maybe, we can have the time to do something that we enjoy and not what someone else wants us to do.