I watched this movie again not so long ago, and honestly, it really touched me. At the time I watched it, I was too young to understand, but now it just clicks.
Greg, by default, isn't a likeable character. He's rude, pushy, self-obsessed and in all honestly, he's not a very good friend. But thats what makes him realistic, thats what makes him a character you can connect with, and also say he's in the wrong.
Throughout the movie, we see Greg wanting to be someone. Wanting to be the smartest, the coolest, the best in the entire school. We see him wanting to be known. And to be known, he reaches out to fit in, to try sports he wouldn't usually try. He constantly wants to look popular, even without the effort. And as this idea gets so much to his head, he begins to change the people around him to be the idea of what he thinks he's like. It hurts his friends, and only then does he realise that maybe he isn't this amazing person, and maybe he can't just apologize to this person he thought was lower than him, and just expect an apology.
Greg goes through the motions of understanding that maybe quality is actually better than quantity, and while he craved to be the best at one thing, he didn't realise he was already best in one aspect. Rowley's best friend.