What does Pearl ask him? Why won’t he do it?

All three hold hands and Pearl asks him, "Wilt thou stand here with Mother and me, tomorrow noontide?" Dimmesdale answers, "I shall, indeed, stand with thy mother and thee one day, but not tomorrow." Pearl persists in her question, and Dimmesdale answers that, "The daylight of this world shall not see our meeting."

I think that, at this point, Dimmesdale has had enough suffering. He needs to "man up" and end the hiding.