Full Idea
If the badness of pain were the reason why we ought not to inflict pain on another, it would equally be a reason why we ought not to inflict pain on ourselves; yet, though we would call such behaviour foolish, we wouldn't think it wrong.
Gist of Idea
If pain were instrinsically wrong, it would be immoral to inflict it on ourselves
Source
H.A. Prichard (Does moral phil rest on a mistake? [1912], n4)
Book Reference
Prichard,H.A.: 'Moral Writings' [OUP 2002], p.10
A Reaction
A very nice point. Note that it will equally well apply to 'benefit' or 'preferences', or any other ideal which utilitarians set out to maximise. It may not be bad to hurt yourself, but it might still be bad to harm yourself.