I'm going to turn down a volunteer job for a paid gig instead, even though I promised I would volunteer.
So I have to ask, what is about the volunteering that you don't like?
It just takes up too much of my time.
Volunteering can be a very rewarding experience. Do your best to honor your commitments so people can count on your later on in life.
Oh whatever, I'm doing work for my own pleasure.
A promise is something you should try your best to keep. Backing out on this promise will have the chance to make you less credible down the line.