"Probably Martha doesn't realise yet how cruel the Doctor is when he goes his length because he doesn't do it when she's there. "

I think Donna was right - he needs someone to stop him. He's classic manic-depressive. He needs someone else to pull him out of the downers (or pull himself out of the downers *for*) and someone to be a reality check when he's off on a 'manic'. And anger is manic behaviour as well, whereas most people think its depressive. Mania makes you creative and hyperactive and think that whatever idea you have is THE greatest idea EVER.... It would probably be a bad idea to piss someone like that off...