Given the circumstances of the time, post-war, and his circumstances, the loss of his wife and stress of the Manhattan project, I might argue a guess that he was grieving.
So it's not just that he found playfulness with physics, it's that he was ready to find it.
What I'm trying to say is don't force it if you're grieving or suffering from some traumatic event. Feynman could play with physics again because he was ready to do so.