People seem to take it a lot more seriously in the united states. Over here, it's not really a clear case of 'You smoke it or you don't', pretty much everyone in their 20's and 30's will smoke dope from time to time, it all just depends on the amount they smoke.
Still, moderation in all things. That might sound odd coming from me, but I don't actually smoke dope that often, mainly because, like....masturbation (couldn't think of another analogy) the longer you leave it, the more potent its effects are. For me, it's once a week, at the very most, typically, it's once every 2 or 3 weeks. If you smoke it every day, you're going to turn into a drooling idiot after a year or two, it's how it works, the brain can't take that much THC. It's the same with drink, you get smashed every day, and your liver isn't going to take it well.
Still, I've never heard of anyone outright changing their personality because of it. It is just a medium strength muscle relaxant, after all. I think what happened to your friends is, they developed some sort of self-esteem problem, and began trying to act 'cool'. The weed is just a part of that. Now that, I have seen before. Don't worry, unless they become total potheads they usually grow out of it.