I'm getting a bit puzzled by the number of threads on here where the OP is called 'selfish' for not wanting to do a favour for someone e.g. taking 3 additional children to school/ babysitting on day off/ taxi service for relative's children/ taking colleague to work. Sometimes these are long-running commitments. Surely when asking a favour the person being asked should be free to say yes or no without pressure either way? I rarely say no to people asking me favours but I would feel mortified if I asked someone for help and they felt obliged to say yes or risk me feeling resentful towards them. I never expect people to do things for me, I just hope that they will and if they can't it's not a big deal, they are still my friend and I would never dream of pressing them to change their minds or to think of them as selfish 
Since when do we need to have an excellent excuse for not doing something? Why is it wrong to say no to something just because it's a hassle that you don't want to deal with? AIBU to think that it's ok to say no to something just because you don't want to do it?