He kind of is cocky. Self assurance as a defence against uncertainty and fear of failure isn't mutually exclusive with being cocky as well. It's perfectly possible and common to be both.



There are several examples across the franchise of Ben not listening to people who know more than him, rushing into things recklessly etc. The fact that he works best with partners to reign him in and balance him out is pretty consistent.



Plus I'd be cautious of using 10 year old Ben as evidence that Ben is not cocky. 10 year old Ben was hugely cocky at times. At one point his recklessness nearly destroyed the entire universe.

Faced with a massive complex bomb plugged into a nuclear reactor his plan for disarming it was...to flip any old switch, pull levers and rip things out at random with superspeed? I mean, that's not just cocky, that's straight up dumb. Advance level dumb.



And then when it goes off weirdly and his Omnitrix starts flashing warning lights he immediately ignores Grandpa Max's warnings not to transform until they know what's happening.



Incidentally in the same movie Ben almost vents himself and Gwen into space by headbutting a random button without thinking.

And then transforms into an alien to chase someone with tentacles on their face, despite knowing that will accelerate the universe destroying countdown and despite this leaving Gwen alone in the middle of a space station full of prisoners where humans are persona non grata.



The point is while Ben is a pretty great hero in many respects he also makes a ton of reckless and cocky mistakes, and frequently acts impulsively in both heroic and mundane situations without thinking his actions through. This is a consistent character flaw of his, especially in the original series.