I'll admit it - I have some sadistic tendencies - I enjoy giving pain - especially to someone who enjoys receiving it. Taking a girl who enjoys pain, making her wetter and wetter with the skillful application is (one of) my favourite forms of foreplay. I will take her to her edge in doing that but not beyond her level of endurance, since that defeats the purpose with the sorts of girls I enjoy most. Essentially, I use the careful and precise application of pain to reduce her to a state where she wants/needs/begs for sex. I use mental pain (humiliation) as well as physical pain for that purpose.
And yes, thrall, I think the unfulfilled need to apply pain is likely felt by both giver and receiver, though probably in different ways (never having been someone who needs to receive pain, I don't know for sure) but there is a sexual tension that builds when the means are not available. Sometimes I use that sexual tension as part of the giving, as part of an extended mind-fk, since the sexual release is orders of magnitude higher when it is finally allowed.