I really like Dexter's response to you. The inability to say no is very often rooted in fear. Such as the fear that the person won't like you or worse that they may be angry with you. Much of this is ingrained in childhood. Parents that punish a child for saying no or not agreeing with them. I believe that it is more pronounced in people who are inherently empathic. You can feel that the other person is frustrated, upset, in need etc. and so feel guilty that you don't take care of them, help them to be better, remove the frustration etc. For some reason, we are also often taught that it is wrong to think about ourselves at all. That the only thing that matters is that everyone else is happy. Then, they'll like us, be nice to us not be unhappy. We have to learn that it is not our responsibility to make everyone else happy and content. In fact that is something that only they can do for themselves. It follows that we must learn to do if for ourselves too. Difficult at best sometimes.
Mark

Though inland far we be,
Our souls have sight of that immortal sea
Which brought us hither.
William Wordsworth (1770–1850)