Tony Stark is arrogant in every MCU movie; it's a key part of his character. In fact lots of male MCU protagonists are arrogant (i.e. Star Lord and Dr Strange as well as Stark).
Tony Stark is arrogant because he is, and has been for all of his life, incredibly, incredibly rich. That makes you believe that you can do anything (or at least, get anything done for you) because, usually, you can do anything with enough money.
Secondly, Stark is an exceptional inventor/mechanic. That's clear from the fact that he was able to make the first Iron Man suit almost solo (only had a little help from Yinsen), not to mention that he was trapped in a cave with only a bunch of scrap to work with [this happens during the first part of the Iron Man movie]. This would also bolster anyone's self-esteem: even without his money, Tony could still make a good livelihood and possibly start up a new company using his skill and a bit of innovation, should Stark Industries fail (which is extremely unlikely).
Stark's heroic successes as Iron Man also probably improved his opinion of his own awesomeness. (This may have been part of the reason for him revealing to the world his secret identity at the end of the first Iron Man film.)
It is worth mentioning, however, that Stark's big ego decreases somewhat as his story progresses in the Marvel Cinematic Universe and he realises that his selfishness causes serious problems for others. (This realisation can be seen throughout the Iron Man and Avengers films, to the point where, by Avengers: Infinity War, Stark's narcissism has almost disappeared - except when he comes into conflict with Strange, since they are both arrogant leaders who don't take kindly to being told what to do.)