Simba is a hero people can relate to: he's not perfect. In fact, he's far from perfect. He is so flawed, that he's more human than well, other human Disney characters.
At first, he a bratty know-it-all. I can relate to that. I believe that we were all, at least at one point in our lives, at a state where we thought we knew everything.
After Simba "kills" Mufasa, he goes through around ten human years growing up thinking that he's a murderer, and he killed his own father. While I don't know anyone who experienced this exact thing, we all felt at one point or another, so depressed and down, that we were borderline (or past the borderline) suicidal. It can be for other reasons: family troubles, bad grades, etc. but we all felt utterly terrible at one point or another.
And when Simba returns, he matures quickly and learns from his mistakes. We either seek to be like that, or are already there.
Not like the Disney princess... (not to bash on them, but the only perk they have in common other than perfectness is naiveness)
Simba in short, is the personification of a perfect being. I define perfect as a person that you can actually become. This includes flaws, but good aspects too. Simba, you stole my heart, mind, and soul.