Simba seems like TERRIBLE King material
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.
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.
However, as an adult in TLK, it`s the Hakuna Matata lifestyle and his immense guilt of his fathers death plus the fear of facing judgment from Sarabi that keeps him from going back. He needs to be reminded that he has to find the strength within himself to take his place as the true king, as that is who he was meant to be. Mufasa is exactly the right person to talk sense into him, and Nala is the one that sets this event in motion by trying to get him to open up about his past.