This is one of my favorite comics I've ever drawn.|
It can be a simple moral story: It's easy to get caught up in your own happiness, hurt others, and thus bring yourself down as well.
It cane be more of a decisive cut at just saying cliched things.
It's also just sensible, since if the power to fly came from the source referenced in it, it'd make sense that you could no longer if you felt bad, and that he starts dropping with his happiness.
Or it can just be a non-senstical set of events. Why can he fly? Why does she take offense? Why does he lose the power then? Why do they feel the need to state all this out loud.
That might just be it though. I like how it sounds when he says "Also I can no longer fly" It's not how someone would say that, but it describes it perfectly, for whatever reason tagging it onto the last statement.
So maybe he did find some pixie dust.