Anyways, back to my main point- character development.
To begin with, there are two extremes of character development that must be avoided. The first is depression. Giving a character a fault that they can't seem to work around or some horrific story is fine, but too much is too much and too much is depressing. Everything in moderation.
The second extreme is suppression of reality. This is when a character is so perfect, so flawless, and so happy that there is nothing left to gain from there story. How can a character develop if they are already perfect in every way? They can't.
Basically, the theme of this is- everything in moderation. It is good to have some elements of joy or at least hope in a character because it give the reader hope that the story will actually go somewhere. It is also good, however, to have some elements of pain and flaw in a character so that the reader has hope that the story can go somewhere.
Remember that development is key to a strong character, but to develop-mental is to watch your story crash and burn.