Vader just wanted results -- along with respect and order. Give them those, and he's fairly easy to please. Palpatine is a sadistic mind-fucker of the first water.

There's natural cheerful, and there is verging on bugfuck crazy. Obi-Wan was just a little *too* enthusiastic about mayhem and odds ridiculously out of his favor.

Plus, it's a little hard to tuck a horse into a two-bedroom walk-up...