[Darknight]

Stephen Bear is very likely a sociopath (or psychopath). Fact: around one person in twenty has psychopathic traits. Narcissistic, lack of remorse/empathy, the (superficial) charm and huge ego all strong indicators.
The masks worn (especially on eviction days), are a huge 'tell'. Bear is expecting to win BB and on eviction night, he feels he should display the same emotions as the other nominees i.e nervousness, hope, fear etc.. in front of the voting public - except he doesn't feel those emotions - and as he thinks the public expect to see those emotions, he covers up with a mask.
There.. a very simplistic explanation from a psychology professor.. you're welcome.