I agree with MeltingHeart that it depends on the situation.
I assume the OP was talking about when someone asks you to do something like go out somewhere with them and you don't feel like going out. In my case, my sister-in-law keeps wanting me to go out with her and ride around for hours in her car while we stop and visit various friends of hers that I don't know. I've told her over and over how exhausting that is for me, but she either doesn't believe me or doesn't care because she still keeps asking me to go. Now when she asks, I tell her I'm not feeling well and leave it at that. In a situation like that, I think it's fine to say you don't want to go and nothing more. You don't owe the person a further explanation.
In the situations like MeltingHeart is talking about, I think it's usually the person who is the most bothered by whatever it is that ends up doing the task, ie washing the dirty dishes, replacing the toilet paper roll, walking the dog, etc. I used to be the one who ended up doing everything because I was the most bothered by whatever it was. When I realized that, I started training myself to let things go. It's not a big issue for me anymore since I live alone now. I can do things whenever I want, or let them go if I want.