I actually really struggle to say no. Obviously context affects it massively (as with most questions like these) but saying no to people seems to be something I have a problem with. While peer pressure is something I often laugh in the face of, I've been told very often: "you're too nice" or "you do things for people too much" or "You always let people borrow your stuff". Which is a result of my lack of ability to say no when people ask for things, of course I have my limits, but it can really become a problem sometimes.