If I try to analyse it (which I believe is what you're after) I think it is often used when someone wants to consider that (s)he is more powerful than the subject of the comment. This is clearly entirely ridiculous, as in my experience there are very few times that you have more power over a man than when you have his cock in your mouth. There's no doubt it's a negative term, but it should be a positive one - it's literally saying "this person gives pleasure" (unless they do it really badly, I suppose, but then I've been told that there's no such thing as a bad blow job).
However, if we were all to go out in T shirts saying "I'm a cocksucker" I bet all the guys would see only the positive elements to it and we'd get chatted up no end!
no subject
If I try to analyse it (which I believe is what you're after) I think it is often used when someone wants to consider that (s)he is more powerful than the subject of the comment. This is clearly entirely ridiculous, as in my experience there are very few times that you have more power over a man than when you have his cock in your mouth. There's no doubt it's a negative term, but it should be a positive one - it's literally saying "this person gives pleasure" (unless they do it really badly, I suppose, but then I've been told that there's no such thing as a bad blow job).
However, if we were all to go out in T shirts saying "I'm a cocksucker" I bet all the guys would see only the positive elements to it and we'd get chatted up no end!