Generally speaking, pokémon that receive a significant boost in power are more likely to lose control of it—and, by extent, themselves. Consequently, while you have pokémon such as audino, altaria, and sableye, all of whom take to mega evolution with very few problems, you also have pokémon such as charizard, lucario, and tyranitar, all of whom are liable to lose control rather easily until they grow used to the transition.
Alternatively, if a pokémon’s abilities are psychic or meditative in nature (see alakazam, gardevoir, and medicham), then they’re less likely to lose control, as their abilities require the utmost self-discipline to use in the first place. Kangaskhan also rarely loses herself, largely because of the nature of her particular mega evolution. (You can read about kangaskhan’s mega evolution here: http://bills-pokedex.tumblr.com/post/152049372796/another-kangaskhan-question-sorry-its-my.)
Conversely, certain mega evolutions simply boost a pokémon’s natural aggression, as in the case of gyarados (which gains the dark type as a result, even), sceptile (which gains the bestial dragon-type), and pinsir (whose aggression can simply be chalked up to the fact that it has just realized it can fly and therefore terrorize mortals from literally any direction).
Finally, you have mega slowbro. Mega slowbro does go berserk, but this is less because of its actual emotions and more because it loses any form of control over the way it moves, as having a 265-pound telekinetic creature with a notoriously slow response time spin about on the tip of its tail is a fantastic recipe for mass destruction.
The Pokédex According to Bill 