um....he had a really bad case of the flu (he was at a show and he was so sick that he played 1/2 the set sitting down and laying down on the stage because he was so drained. The other half of the set he did not take part in), then, when he had something go on with his tooth something happened with his nasal and he had to fly back to the US to get it all checked out and operated i believe, and before TBP he had mono during the 3 cheers era, also he had some personal family problems that he had to tend to so he left tour for that also.
wait! now that I think of it, I remember that Josh did say he was a bit of a serious person. He's starting to remind me of Mikey Way about how he performs onstage. Mikey is a very quite guy and isn't as energetic as Frank, Gerard, Bob, or Ray but he does love to perform. So maybe it's just his personality. Everyone's different