Every main character needs to have both outer and inner conflicts. Outer conflicts are the people, places or things that the main character struggles with, while inner conflicts are the personality traits that the main character struggles with. For example, a main character might hate the guy who sits in the next cubicle (outer) and he may also hate that he has never been successful enough to work in a real office (inner).
In his Writing the Breakout Novel Workbook, Maass has a three step exercise intended to help create stronger inner conflicts. For the first step, he suggests you define your main character’s goal. For example, in one of the Princess Diaries books, Mia wants to go to the prom with her boyfriend Micheal. This is her inner goal for the novel. The second step is to write down the opposite of the goal. In the case above, this is Mia not going to the prom. The third step is to create a scenario where the main character wants both the goal and the opposite of the goal. Using the same example, this means that we need to see Mia both wanting to go to the prom and not wanting to go. Meg Cabot (the awesome writer of this series) accomplishes this by giving Mia a boyfriend who detests the prom. Mia wants her boyfriend to like her so she ends up both wanting to go and wanting to please her boyfriend (which means, not wanting to go).
I’ve simplified this exercise a little, but the point is that, if you make it a struggle for your main character to achieve their goal, they will have more depth and thus be more interesting.