Flowers for Algernon question

I think it was because he had no real personal, emotional attachment to Fae so perhaps it was less pressure to be with her, because it didn't mean so much to him. ALso, whilst Alice had known him before he had become "smart", Fae hadn't, so maybe Alice reminded him of the insecure, unsure, unknowing and oblivious person that he was. Maybe this was too hard for him to endure in that intimate situation, due to the dramatic change in their relationship, from almost maternal to romantic.