I think this is just a good example of how the TOS characters, while living in a more advanced world than ours, were still fundamentally human. Kirk was a great leader, but he wasn't perfect. He had his insecurities and his flaws just like everyone. And that makes him all the more believable and enjoyable to watch.
I think throughout Star Trek we've seen plenty of evidence that one of Kirk's greatest insecurities in life is losing command. He doesn't know anything else. He doesn't feel fulfilled doing anything else. And he thinks that he would just grow old and be useless. We saw that in this episode, but we also saw it in The Deadly Years, in TWOK, in Generations, etc.
I think his behavior here was perfectly reasonable for the character. Here's a man who's ordinarily a great leader, but when confronted with one of his deepest insecurities -- the fact that something might come along to replace him and rob him of his command -- he behaves in a very human manner.
This isn't a bad summation. However, the biggest example of Kirk's 'need' to be in command was probably in TMP in the way that he snatched command of the Enterprise right out from underneath Decker.