Jack is quite possibly the most excited anyone here has ever seen him. Yesterday, he was prepared for the absolute worst to happen (probably everything here imploding and everyone getting left braindead in their bodies) and now, today? They have a second chance, a chance to do everything they didn't do the first time around, a chance to live and be happy, wherever and with whomever they want!
It's more than he ever really expected to get.
It's tempered by the fact he's going to have to leave so many of the good friends - and adoptive family - he's made here, but even that can't get him down. He'll miss them, but he'll move on as he always does, and he'll hold them in his heart along with everyone else he's lost.