And he's left from having a TARDIS brimful of all his friends, brimful of his new family, a few hours later he's on his own again. Journey's End is almost (almost) worse than Doomsday. Not even because he's lost Rose again, but because he got everyone back, everyone he cared about, everyone who helped him survive, and lost them all. It's no wonder regeneration wasn't long in coming after this. I don't know how he survived losing everything twice.