Some thoughts about Greg:
He's said before that his mother was extremely over-protective and wouldn't let him play sports when he was a kid. So growing up, Greg probably didn't really get to be kid because of his mother's over-protectiveness. He didn't get to do a lot of what other kids got to do, which means he probably didn't do much more than study and all of that.
I almost get the feeling that since Greg was over-protected, he was more innocent and it sounded as if as he child he was extremely sheltered (he did say he didn't become fully intimate with anyone until he was 22 which is obviously due in part to his mother's over protectiveness). So being more innocent like caused him to look at the world with virtually innocent childlike eyes (almost as if he was seeing things for the first time) and sometimes with curiousity not unlike that a kid would have. Maybe that's the reason he used to be kind of upbeat, almost always smiling, etc.
After the explosion and the beating however, he didn't see the world the same way anymore. I think where before he may have had more of an "every day is an adventure" type of outlook, now it probably seems more like "every day is a struggle" or maybe a little of both (which is really the reality lol).
Even though I love how Greg has matured, I'm glad that this season, the writers have written some nice, happy, fun scenes for our beloved Greg. The bowling and the Henry's birthday episode, etc. But I do feel like the things that happened to Greg and the things he's seen that people have done to each other has hardened him somewhat ... and this may be why he seems more serious a lot of the time now.