Simba is a kid and acts like a kid would. Especially with the knowledge that he will one day inherit an entire kingdom. And thats fine. Mufasa attempts to impart wisdom to him, but with little to no effect. At one point he even says to Simba "Being king isn't about always getting what you want." Simba never seems to understand this.
After being repeatedly shamed for his abandonment by his lady friend, which she was right to do, he decides to go back. The main motivation for this was his own greed and selfishness (granted this is also what motivated Scar, but he acted on it directly). Simba still thinks being king would be neat and put him in the center spotlight.
In short, Simba is a pussy. He has absolutely no growth. Simba is a coward who runs away from his problems from beginning to end in this movie. Remain a child forever, run away from conflict, and you'll get everything you want. Is that a good message to send to children?