I’m with @livelaughlove21 on this one, for the most part. If you’re close enough with this person to have open and honest communication (without the person feeling like they’re put on the spot because of it), I suggest just asking if they’re still alright to do it, and that if not, you understand. That way you’re letting them know you sense their possible hesitation and you’re also giving them an opportunity to be honest if they’d rather not, for whatever reason.
If it’s bothering you that they’re only going to help because they’re obligated, and not because it’s something they’d genuinely like to do for you, I also understand being a little upset about it. Give them the chance to explain, though. Their reason for hesitation might ease your mind somewhat if you are feeling hurt. If not, completely disregard this part of my answer. Heh. :)