That is because you're only looking at her as the hero of the show. Believing that her role makes her free of making mistakes or being wrong. A flaw that a lot of people make when watching cartoons.



Star pretty much proves that she loves being the center of attention the way the flaunts her magic all willy-nilly despite the fact that she knows that a simply misspell could burn the city to the ground. Heck in the fist episode she did burn a city to the ground but she feels no guilt over it nor did she learn any sense of remorse as she still shows off her magic to Marco nearly burning down the school.



As for her being selfish, she showed that during the banagic wand episode, how she wanted Marco to drop what he was doing just so that she could alleviate her boredom. Proving that she gives very little regard to other people's priorities and responsibilities. That having fun is the only thing that matters to her. Then at the end when she denies Marco his banana pudding, that shows that Star can't stand people looking at her in a negative light. Which is again why she tosses her magic around at school to maintain her popularity. It's the same mentality to when a rich kid uses his money to buy friends.