Dipper was mesmerised by Pacifica's perfect form. If we wasn't in love with her before, he was now.
Not a day went by when he didn't ask himself the same question.
"Why? Why is the most beautiful woman in Gravity Falls, no, the whole world, no, the whole multiverse in love with me?"
Of course he wanted to ask her, but he was worried that she would question it herself and that would drive her to leave him.
It worried him that he used to hate her, even to the point when he told her that she was the worst to her face, before he saw the real Pacifica.
Of course, it was Dipper who brought out the real Pacifica in the first place. He was the one who set her free with the tyranny of her parents and she would love him forever for that.
There's plenty of things he's not gonna believe over time, such as Pacifica being his future wife and having a nice family (maybe even a big one since she looks voluptuous here) with her.