When Danny had played down, he had no clue that Dan was in his room. Dan had read the love potion bottle, and it said that he can alter thoughts too.
When Danny was fast asleep, Dan hovered over Danny's head, and stuck his hands in his head. He started to change Danny's mind.
Danny's new mind consists of, him being gay, hin loving Dash, him being a bottom, and hin having multiple fetishes.
After that, Dan left and flew off to Dash's house.