Having played through for the first time in some while:

Braid is about emptiness and a driving need, and about the effects of following that ambition — of constantly chasing some external ideal rather than constructing one’s own happiness.

Which ties in pretty well to the game’s philosophy and attitude toward game design. And also into his reluctance to objectify the game, to tell people what to make of it.

In typological terms, you could say it’s about the conflict between Ni and Te.