This thesis would explain why so many marriages fail. When people marry in extreme joy, extreme romance, high passion, they’re making promises in a moment when no promise will hold, according to this theory. The violent joy leads to setting a marriage in motion, in enacting an enacture, and this very consequence, destroys with joy the joy.
But of course that’s not what the king is suggesting. He is not suggesting that his own marriage will be destroyed by violent emotion – just that grief might cloud a person’s promise making ability, that it might make misdeclarations a little suspect.