As a cub, he was very adventurous. Sometimes, you just want to explore all on your own and go to mysterious places (even if u can't). Even though that he was told not to go to the elephant graveyard, he went anyway and took Nala. Later, Mufasa tells him that he delibertly disobeyed him. Simba didn't mean to disobey his father but it his adventurous personality that made him go.
In SP, Kiara asks Simba if he will let her do her solo hunt on her own. Simba was hesitating because he didn't know what to say and Nala gave him a playful warning look to make sure he says yes. He then promises but sends Timon and Pumbaa to watch her again since he didn't want anything bad happening to her. Later, Kiara got mad at him for a second, claiming about him breaking he promise and he says to her "It's a good thing I did. I almost lost you. No More hunts for you! Not Ever!"
I would say that 50% percent of Simba's promises that he broke were just making mistakes, while his other half were because he had no other choice.