Personally, I don't like Jim Kirk in the movies, dating back to the first one. This is a guy who went with his gut, but he didn't outright defy people in advancing in rank to become the youngest Captain in Starfleet history. It isn't until he's sitting in prison in TUC that he decides "Hey, I don't get to do whatever the hell I want. I'm not right all the time."
How do you know this? When were we ever shown Kirk at 25, this early in his career?
Never really. At best we've heard what others thought of him prior to assuming command of the Enterprise. Mitchell said he was a stack of books with legs. Carol described him as "never a Boy Scout". Both might be true or perhaps the truth is somewhere in between.
As a Captain, Kirk often came into conflict with his superiors and he did defy them when he felt it necessary. I doubt this was trait that developed after he gained command of the Enterprise.