Isn't it a fairly simple question?
Why do I seek submission? Because it turns me on. If I wouldn't be arroused by submitting and all the things involved, I probably wouldn't do it at all. Why do you think it is important to discuss between D and s what the sub is willing and not willing to do? He/She will not easily do those things that don't turn them on in any way. For me it's as simple as that.