It is difficult to make someone into something they are not... whether that someone is yourself or someone else. You need to both be enjoying what you are doing. That doesn't mean that a slave or sub has to enjoy all of her/his punishments but she/he has to enjoy submitting to them at least. And if she/he is getting pain for pleasure I would hope it would be pleasurable. If it isn't, then maybe it isn't something she/he should be doing. Just as if giving or administering pain bothers a Master/Mistress or Dom(me) maybe they should be doing other things. I think both parties need to be totally comfortable and I do think that is of utmost importance. If you are doing something just to please someone (even in a vanilla relationship) rather than being true to yourself it always has the potential to be disasterous.