John has progressively gotten more obsessed with Alex over time and I think that maybe a little of is the fact that he didn’t instantly get what he wanted for a change. Not getting what you want tends to make some people want it a lot more than normal. Although in this case an element of realization may also be playing a part. It really sucks when you think you have all this time to do something then suddenly something snaps it all away. It gets to you psychologically. Additionally Alex has her own agency, so there was no guarantee she would have agreed at the end of it all anyway. Obviously she was interested, but after spending time with John she may have cooled to him over time. Maybe he isn’t the person her fantasies made him out to be. Sometimes the people we think we want turn out to be not what we want upon closer inspection, and the reverse. Sometimes it seems amazing anyone ever manages to couple with anyone else.