In episode S02E04 of Game of Thrones we (and Tyrion) learn that Cersei Lannister has an affair with Lancel Lannister. Seeing that she doesn't seem to get any advantage out of this apart from the obvious (rather on the contrary, he actually becomes a security vulnerability for her when Tyrion blackmails him into spying on her), I wonder why she did so?
Did she want anything else from Lancel than mere sexual enjoyment? Was it a retaliation for her real love Jaime, whom she seemed to hold responsible for his absence (as we learn in S04E01)? Or maybe it even was a "payment" for Lancel's contribution in Robert Baratheon's death. If she didn't want anything from Lancel in particular, then why him? Was this just a question of availability or did she have a deeper reason for this choice, like an inherent attraction to her own kin out of some kind of narcissism or whatever? So, why did Cersei sleep with Lancel?