It was probably the hardest thing Nine ever did, asking for the key back. He had to act like it didn't bother him in the least to run off without her- when in reality, it tore him apart. <-- one of favorite episodes.
This is one of those things that has always bothered me. It got a little better when Ten talked about each regeneration being a new man even though he's supposed to be the same man, but still. I wondered why he forgot about Nine.<<< I noticed this too!