If you look at the early seasons of Mythbusters, when the the show used to be unscripted reality, you can see him being far more comfortable. He's playfully joking around with Adam and you can see how he sees a lot of potential in Kari and makes sure she's involved and act's kind of like a mentor to her. Watching the early seasons, I forgot how different Jamie used to be.

I think when the show became scripted reality, you an see Jamie feeling uncomfortable because it relied on him having to act. Naturally, I think he developed a character that's mad-scientist-esque to oppose Adam's character and I think he sort of forgot who he was or the character he was playing took on a life of it's own and he was assumed to be exactly like the character he was playing instead of his real life persona.

It must also be hurtful to hear Adam say that him and Jamie aren't friends. Jamie essentially gave Adam that job and Adam would be nowhere near as popular without Jamie. I think maybe Jamie wanted to concentrate on the builds and the engineering more but Adam wanted to make it an 'entertaining science show' and that's where they clashed. I also think the producers changed and were probably more in line with how Adam wanted the show to be and Jamie probably got labelled as being difficult because he was the only one with that particular opinion.

If you look at the reunion episode, you can see that Kari is so thankful to Jamie for giving her that job (and Tory even says his thanks too) but Adam doens't really seem to acknowledge Jamie much at all. I think after around season 4, Adam's ego became bigger and he started to think of it as his show and wanted his own way all the time. At the end of the day M5 was Jamie's shop and Jamie was asked to do the show. I have to say, I kind of feel sorry for him.