At home, he's bullied by his elder brother. And he resents the adoration by his baby brother. You see, he's in-between and he hates it. In middle school, he's one of the youngest, the shortest. And he's one of those bullied. It's a terrible feeling, that. To be bullied, I mean. You get to the point when you question your own existence, not the bullies'; when you wanna get back at them, but you can't. You wanna be in, you wanna be popular, but you don't know how, because you are still unaware of what you can do.
That is Greg. He wants to be popular in school, in that microcosmic world he's in. He has ideas in mind, and decides to implement them with the help of his bestfriend Rowley. But the things that he does manage to go wrong all the time, so it seems. So wrong that some other fellows get hurt, including his best friend who dumps him at one point. It was his fault, actually, because he tries to save himself at the expense of Rowley. In this movie, he is portrayed as a self-centered little boy. I think it's a general truth that kids at this stage, because they are going through the changes, could only think of themselves.
The movie is not entirely about Greg, the little boy who wrote the diary...well, the journal, as he insists: it's also about his bestfriend, Rowley, the chubby boy who used to be left out of the crowd because he's who he is, but gained popularity because he's simply being who he is. Greg tries so hard to become popular but ends up getting himself deep in trouble; Rowley gets in trouble because of Greg but manages to redeem himself. Just like when Rowley broke his arm in an accident that was caused by Greg, though the latter didn't mean for it to happen. The thing is, Greg didn't even apologize, but Rowley is a faithful friend: he didn't mind. Rowley gets the sympathy of the other kids in school, especially of the girls, and Greg tries and fails to veer their attention to himself. He ends up being scorned for what he did. But for Greg, he believes he created Rowley.
Rowley is a gracious kid. He's chubby, a fact that makes him a laughing stock in school. But he accepts himself for what he is, and tolerates the mocking. After all, he can't help being himself. What makes him adorable is that he is a loyal friend. He accommodates the whims of Greg, and stands by the latter. He takes it all in, until he could no more. So one day he drops the bomb and tells Greg off, finally ending their friendship. He no longer wants to be stepped on, especially by his bestfriend. Rowley moves on and finds other friends. Greg finds himself alone. Rowley doesn't have issues with himself, despite himself. Greg has everything in his teenage mind going in turmoil. Well, it all boils down to wanting to be accepted. Problem is, he cannot seem to accept his brothers, his mom and his dad, even himself.
But then, the story does have a happy ending. Greg and Rowley gets back together again. But not without some good realizations of things about life. Change in outlook is imminent.
The theme here is: Some people exert so much effort to be liked yet end up not being liked. Another is, some people do not realize who their real friends are until they come up in tight situations.
The lesson here is, Life can never be as perfect as you want it to be. So be true to your friends, be true to yourself and everything will be alright, never mind that life's not perfect. It will never be.