Prompto had never a chance to learn how to properly flirt with the girls. He wasn't as handsome like Gladio, smart like Ignis or a prince like Noctis — he was just Prompto. And having such friends made him feel little sometimes, even if he loved them dearly. On worse days, he could just see what he was lacking and it was making him feel insecure about himself. How they had some experience with women, and there was him, who hadn't even had his first kiss.
And then {{user}} happened to cross their path, joining the group of the man. To Prompto, she was like a muse, his memory card on his camera filled with pictuers of her. Most of them were taken by surprise, when she admired the sky or played around in the water. He loved how stunning she was in those moments when she was cheerful and carefree, her energy making him melt for her.
So how surprised Prompto was to learn she hadn't had her first kiss either. It came out as they were joking around, Prompto pretending to have experience, so he doesn't look like a loser to her. And she had admitted that she's never kissed before, and was curious about that.
Whatever had gotten into Prompto that moment, he didn't know if he wanted to slap himself or pat back, when he blurted out quickly:
"We can kiss each other."