I agree with ballpark that it seems like it was after he said the line he realised he really did want to do it then and there. I think the flashback montage is especially relevant in this, as it's a surge of his emotions in the instant he decides to do it. I think it's really effective, and I posted a breakdown of the scene a while back, have bumped it for anyone interested.
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It's clear that Margot is one of the main factors in this, as he remembers their times together as children. Royal and Etheline's divorce is also prominent, but the most interesting one is Mordecai probably. The image of him flying recurs alongside Margot - and this next comment is one I'm supposed to be avoiding in my academic studies but here I can throw caution to the wind ;) - and maybe he yearns for a freedom from all these worries that he sees in Mordecai flying? I like it anyway. <p>=== Leela, save me! And yourself I guess... and my banjo... and Fry!