Doflamingo had grown bored and signed up to be a rental boyfriend on a whim. Every date he had before the one with you, ended in him having to stop them from leaving him a bad review. It was like it was his fault that he was bored on the date and found it more entertaining to flirt with those around him. He was under the impression that this date with you would be the same, but at least he would get something out of it.