I like to see each Doctor as part of a whole influenced by the nature of the regeneration. It seems implied from Romana that a regeneration is a choice someone makes at a certain time for a certain reason. And then they pick a safe spot and regenerate and then take time to recover. Whereas the Doctor seems to literally die each time and then get thrown into an adventure with no time to adjust.
2 was a younger bouncier guy who was able to enjoy life after 1 aged into a cranky old man.
3 was probably the action hero because of the way 2 was forced to regenerate, he wanted to be strong.
4 was more alien and quirky as a reaction to the time stuck on earth.
5 was more friendly and emotionally vulnerable because of the length of time 4 had and the companions he'd made (he wanted to just enjoy life again).
6 was an erratic reaction to the poison that killed 5 or maybe just too smart for his own good. certainly i believe his douchey attitude was bitterness at having "died" to save peri.
7 was stable, controlling, and cunning perhaps as a reaction to the unbalanced nature of 6.
8 was young and strong as a reaction to the very agonizing and drawn out death 7 had while in a weakened state. also he was more "human" and emotionally open perhaps as a reaction to the manipulative way 7 became.
9 was a basketcase of emotions because of the trauma of losing his people and the war.
10 was made for rose. she had brought a sense of fun into his life and he was all giggles with her in series 2 but then displayed signs of depression in 3/4.

11 seems to have moved on and let go of his emotional baggage. there was a line in Eleventh Hour about being lucky he was the last. I think the End of Time and the way his people wanted to destroy everything to survive mixed with saying goodbye to everyone allowed him to be happy again without clinging to anyone. he's also clearly less interested in forming longterm bonds with humans, a lesson he learned from 10 and i noticed strongly in the way he pushed rory and amy together because he knew she'd have to leave and he was cool with that.

That's just my theory anyway. A way to make sense of the various actor's takes on the characters in a hindsight is 20/20 sort of way. But some of it makes sense and seems to be a subconscious reaction during the regeneration. Like how Romana picked her form, maybe the Doctor tries on some level to push who he'll become. But then certain aspects are always there: good guy/hero, explorer, etc.

Bow ties are coool.