Greg is a sociopathGreg rarely shows empathy for anyone but himself, and almost never feels sorry for all of the obnoxious and/or cruel things he does to people. He also constantly manipulates his family and "friends", especially Rowley, and one gets the impression he only sees them as tools to facilitate his own gratification, for example, he constantly takes advantage of Rowley's childlike mind, tricking him into doing things for him, or taking (or at least sharing) the fall for something that is entirely Greg's fault. Greg also seems to be rather smart for his age, he can read people easily and play on their emotions and personalities. He probably WILL be successful when he reaches adulthood, in Politics or some other field that requires ruthlessness.
The books are an exaggeration, and the movies are the true storyThe books have Greg be less of a Jerk Ass and makes the tales of abuse even more abusive. He also cuts parts he doesn't like (like getting beaten in wrestling by Patty Farrel). The movies are the true story of what happened. Example: In the book, Greg frightened the little children with a worm. In the movie, Greg says that aliens are going to eat them, and abandons them in a construction pit. IN THE RAIN.
The books are actually complex social commentary on the state of America's educational system.Yes, a teacher not knowing how "jolly" is correctly spelled and an eighth grader not knowing how to carry in math could both be chalked up to Rule of Funny, but some of the events in Greg's school hit too close to home for many students for it all to be exaggeration.
Manny is autisticThe whole hot dog issue in Cabin Fever, plus he doesn't seem to be maturing. Not to mention he's young but smart enough to turn off power in the entire house except his own room.
Greg's Dad spent the blizzard in the hotel with his mistressDunno, it just seemed odd that he would say he couldn't get home.
Greg (in the movie at least) was the one who wrote on the doorThe handwriting on the wall matches up with Greg's. In the book where it shows a cartoon he drew of the incident, this can be easily explained as artistic interpretation. In the movie, however, the only explanation is Greg did it. Perhaps something illicit was brought to the party and Greg imbibed. Under the influence, Greg started to think his brother was a really cool guy and thought it would be nice to write "Roderick rules!" on the door in black magic marker. He has no memory of this event.
Frank (Greg's dad) thinks that Greg is gay and does not approve.Greg acts really feminine and flamboyant at times; caring for a baby doll as if it were his own child, saying that he'd choose water jazz over swim team, not liking sports and other "manly" things, etc. Perhaps Frank suspects that Greg is gay, and is trying to "stamp it out of him," hence why he throws away said doll without asking, refuses to listen when Greg expresses dislike of sports, tries to send him to military school, and other things like that.
Manny is actually a very smart kid.He's definitely smart enough to know how to work the electricity in the house and appears to know that his mom accepts "I'm only thwee!" as an excuse. He's simply Obfuscating Stupidity and pretends to not be maturing because he knows that as long as he acts childish and throws tantrums, he'll always get his way. Manny, you Magnificent Bastard.
Manny has OCD.Think of how he wanted his sandwiches cut JUST RIGHT, and how he wanted his hotdog to have the mustard in a PERFECT LINE.
In The Third Wheel Greg and Holly will be an offical couple.In the third movie that's what happened so it doesn't seem unlikely.
In The Third Wheel Greg's Date To The Dance Will Be PattySomehow, he'll either be really desperate or forced into taking her and will have a hilariously terrible time. As consolation, the author will have him hook up with Holly afterward (in line with the above guess).
In a future movie Bryce Anderson will be the main antagonist.He is featured in the past three books and the third movie ends with Greg and Holly becoming an official couple. Holly herself isn't part of the main plot in The Third Wheel but it is mentioned that she likes Bryce in that book. Now in line with the WMG that the movies are the true events and the diaries are exaggerated, it could be that in the films universe Greg and Holly where not in good terms in their new relationship, so he didn't write her in his diary at the time. If a movie adaption of The Ugly Truth Cabin Fever or The Third Wheel is made Bryce would probebly be a Romantic Rival to Greg.
Holly & Chirag are actually Emma & Ravi.Chirag's rivalry with Greg over Holly is actually an example of My Sister Is Off Limits… though it doesn't explain why Holly is quite a bit smarter than Emma.
The Last Straw was supposed to be the last book in the series.It ends with a happy ending and the last page says in large text "THE END". Furthermore, the next books try to stretch shorter and shorter timeframes into a 200-page graphic novel. Case in point: The Third Wheel takes place over several weeks, yet is just as long as the first book, which takes place over a year. Perhaps someone at the publishing company saw how well the books were selling and decided to make the author write more.
The series will go on for a really long time.The series will officially end when Greg and Rowley are in college.
Related to the first theory, Greg has clinical narcissism.He shows quite a few signs of narcissism, particularly as the books go along. They include but aren't limited to: