Cub: He is proud, arrogant, and very cocky. Even Zazu remarks that he'd make a bad king.
Cub - Adult - he spend years under Hakuna Matata and pretends his problems are all gone. When Nala comes and asks for help, he blows her off too and shirks his responsibilities. It's only his own father that got him to go back.
King of Pride Rock - He smothers Kiara and is way too overprotective. Also, he never gave Kovu the benefit of the doubt (despite himself having been falsely accused and knowing how that can feel!) but exiles him, which causes Kiara to run away. If he'd asked Kovu what had gone on and if he'd been plotting with Zria the whole time, he'd have given Kovu a chance to say that yes, but that he'd come to his side due to Kiara and that he hadn't helped Zira attack him in the cave. Simba might have thought that over too and not exiled Kovu.