Flying brings out the best and the worst in a personality. A well formed, non egotistical, non arrogant type could be a great pilot as long as they keep a level head and don't let the perceived "prestige" (lol!!) of the job go to their head.
However, in the younger formative years, its easy to let the job form "you" as a person.
I've flown with some great guys, but I think its only fair to say that I have also flown with some total megalomaniac idiots where the flight deck gradient is ridiculously steep to the point of being un-workable. Some of these have been the more "senior" guys who have been in the industry for years (or dare I say have come from ex-RAF!!) who see no reason to adapt to modern CRM methods....