In-universe: I've been thinking Kelvin Kirk would have been better off being some kind of special ops soldier that Starfleet could throw at situations. He was great in taking down Nero, he succeeded in getting Khan, and he took down Krall. But he sucked at following Pike and Spock's orders, he mishandled Nibiru (according to Starfleet), and then a few years later he was already bored of being a captain and wanted to become an admiral. I think he would have made a good MACO.
2255: "Your father was captain of a starship for 12 minutes. He saved 800 lives, including your mother's and yours. I dare you to do better." 2263: "My Dad joined Starfleet because he believed in it. I joined on a dare." Kirk was never bored of being a captain. He was disillusioned and questioning his place in the universe. There's a HUGE difference. By the end of Beyond, he realised crazy space adventuring was what made him happy.
kirk loves to challenge himself. it was all about the challenge, this is seen throughout the film when he cheats so he can beat the Kobi Yasi Maru test when he bugs uhura for 3 years to tell him her first name.
He failed a question in "Truth or Dare" so he had to do the dare. What the question was, that's anyone's guess... (Being Kelvin Kirk and not TOS Kirk, of course...)
Pretty much sums it up. Kirk was rudderless prior to meeting with Pike and then took on the challenge Pike laid out for him. Kirk was living in the shadow of his heroic dad, imagining what his dad might have been like.
Which I thought was interesting, since in four out of seven prior movies we began with Kirk chafing against his decision to stop captaining (twice knowing he missed it and twice having to be pushed out of his rut by Spock and McCoy), but we never once saw even a hint of why he'd give it up in the first place until Beyond.