Actually, the reason I've discovered for Bob's mood swings is because he likes competition and doesnt like easy races at all.
For example, I used one of my Bobs in the Red Bull X 2010 to move along my B-Spec trophy acquisition and other events thinking fast car, less wait time, sure gold trophy. Seeing how I paid my dues for six years playing GT4, Im not going to redo ever simple race that I did 20 times in GT4.
But I digress.
While Bob was winning easily and I was getting him to do what I needed him to do, his arrow kept pointing down. Apparently just winning for Bob isnt enough.
Later, in some of the races where you cant use the X 2010 I just used over powered cars to win B-Spec events with the same results for a different Bob. Finally, I took one of my stock cars and raced it because I didnt feel like buying and modifying a car to compete in a race. Bob LOST the race by a slim margin but to my surprise, Bobs mood arrow leveled out. I added better tires and raced again. Bob barely won and his arrow was now pointed up.
Every time Bob ran a close race, win or lose, his attitude arrow stayed up. Thats the key to Bobs mood: let him RACE and he will be a happy little driver for you.