It had been a few weeks since the Doctor had wiped Donna's memories. He was on his own. He felt like it was better it stayed that way even if he was lonely. He felt like he ruined his companions lives and that it was his fault.
He felt like he was always the one to cause something bad to happen to them, or for things to just be too different.

Walking alone the Doctor looked around and he seemed to be thinking. He wasn't sure what he was going to do now.
All he could do was travel alone. It was a cold day and he wandered around London with his hands in his pockets. The Doctor was truely alone now with nobody he could have by his side, he knew it was best to leave his friends who still remembered him and lived in this universe to live their own lives