I think it's pretty obvious that Kylo uses the mask more as a way to hide his insecurities and instill fear and intimidation in others. He wants so badly to be like Vader but knows the truth that he isn't, so he goes so far as to use a literal mask, which can be interpreted as a metaphorical mask as well, to hide behind.
thats the only justification i suppose. but vader took his off when he went good, no? kylo should of been like "well.. i killed off my last connection to the light side, helmet up like grandaddy"
but in my head, i'm thinking... it's cold as fuck, you're about to fight. take your helmet.
whatever, it's just a helmet. good movie.