Panel 1: We start off with Elly washing the dishes; as she does so, John says "Since you want to work, honey, I have a suggestion."
Panel 2: As she faces him, he says "Jean is taking the week off so I need someone at the front desk."
Panel 3: He then says "It's not exciting but it would be a change"; this sparks something within Elly.
Panel 4: As she hugs him in gratitude, he thought-bubbles "What have I done?". It should be noted that his face is frozen in the Bug-Eyed Glare of Existential Horror.
Summary: This is because he probably didn't expect her to take him up on the offer; it seems to me that he thought that her need to escape the prison she made the Pattermanse into would blow over. Too bad for him and everyone else that it happened after Kortney made a fool of her; by then, of course, he was scratching his head and possibly wondering what happened to her dreams.