2. Regeneration
Let's face it, if you had so many changes of identity wouldn't it just make you a little bit unbalanced? The poor dear really cannot be expected to have a clear handle anymore over where he ends and the rest of existence begins. What's supposed to happen, I guess, is that the previous incarnation gets packed away neatly into a corner of The Doctor's memory. At the very least in the cold light of day he must remember something and ask himself, "Did I really do that?" You know, like when you remember yourself as a kid having a tantrum over something like your dad refusing to let you have a go on that one minute space rocket ride that used to stand outside the supermarket door precisely so that it would entice children to pester their parents into parting with a few more coins before going home? No? Just me then... It's bad enough if you can confine yourself to only remembering non-threatening-cold-light-of-day-impersonal-memories, but what if it involves something really embarrassing, or traumatic? What could that do to you? And, what if there are nine, ten, or eleven, or more personalities packed away in there? At least I only have to squirm and own up to one of me, but if you have spent a lifetime being something other than what you have now become then that's going to prove pretty stressful I reckon, which brings me on to the final point...