Sentinel was once one of the impulsive-types until a disastrous "unsanctioned" mission he'd suggested with best friends Optimus Prime and Elita-1 ended in the loss of Elita. Though both 'bots grieved, Sentinel blamed Optimus for abandoning her. He placed his anger on his former friend and drove himself to climb the ranks into the Cybertron Elite Guard through strict adherence to military protocol. The end result is someone Autobots' greatest assets but is such a jerk it's hard to see how