If I can put up my psych major hat for a second here...
I honestly don't think Portnoy is egotistical; I think he is a very insecure person. His actions, the way he talks, the way he acts, just screams of insecurity. It's why he does so many little things for the various bands he's in, always looking for a way to please others so they like him. Same with wanting the credit for doing such things. By getting credit, he gets props and makes people like and appreciate him. Or the fact that both his FB page and the home page of his website are draped in drum magazines where he was on the cover; he's not doing it so much to brag, but to remind everyone, "See, I am good." He needs that constant validation, which is why he loves his hardcore fans so much, because they constantly validate him. Meanwhile, an insecure person doesn't take criticism well, even when presented in a constructive way. A secure person simply doesn't react so negatively to criticism the way he has over the years; a secure person can blow it off and move on since they know that not everyone is going to love them and/or everything they do.
Now, I do think he had gotten a little too big for his britches in the late 00s with Dream Theater, thinking he ran the show and they could never go on without him, hence him leaving the band and publicly saying so almost immediately, when most people would have waited for the smoke to clear before telling the public. I am guessing he figured his value at that point was so high that public pressure and everything would persuade the band either that he was right in needing a break or that they could not go on without him. Plus, he thought he was a good bet to keep the drumming gig in A7X full-time and long term, so he was covered either way. Or so he thought. Turns out, DT said, "See ya, we're moving on," and A7X said, "Thanks for the tour, adios." And he was left with neither. That would make most people a little bitter and resentful, especially an insecure person who frequently needs validation.
As one last point, I have said for years that he is almost a different person when he is around Neal Morse, and that is easy to figure out why. Neal is probably his closest friend and biggest musical soulmate, one who accepts him no matter what, so he is comfortable, relaxed and laid back around Neal, because he always has his best friend right there with him. Really, it's no wonder he does so many projects with him. It's more than just the music aspect of it, which is strong already.
I am sure this breakdown will go over the heads of some...