A pain sponge is a very strong person. They enjoy being vented to and "soaking up others pain" because they are extremely sympathetic. They usually never express their negative feelings, or tell people their problems, because they don't like to worry people. Pain sponges live for the sole purpose of making people happy. It can be dangerous to be a pain sponge or emotionally close to one (i.e. close friends, lover, etc.) because once they soak up enough pain (I note that it is a very high amount) they will have a breakdown, or the sponge will "squeeze the pain out" of themselves.
I just so happen to be a pain-sponge. I listen to other peoples problems but never share my own, because I wouldn't want to worry them.