No one wants to give up good sex. If there was a good physical relationship, it's a good reason for them to return... especially if they know that you enjoyed the sex, too; that's their "in" (so to speak). The problem with great sex is that it can create a false-positive on the happiness of the relationship. When it comes to broken relationships, great sex isn't necessarily due to physical compatibility, but to different feelings and situations that mask the nonworking parts of the relationship: making up again after an argument, renewing the rush that accompanies seeing them again, and the like.