there’s a difference here. ariel had limited first hand and wide second hand experience of humans, given her collection she must have gained SOME knowledge of the human world and come to understand that her father was wrong about them being mindless idiots. ; secondly, let’s take the love-at-first-sight into account. I know that people believe it doesn’t exist in real life, but in Disney movies it does and in Disney movies it IS logical. Ariel had love at first sight with Eric. When you have LAFS you know that that person is meant for you, and that’s that.  Rapunzel didn’t have that with Flynn. So hers was just stupid. Seeing the lanterns isn’t- she KNEW they were meant for her. So, that’s not bad. But leaving with Flynn was.