Personally, I feel that A-Rod had done all he intended to do as a SS(he was regarded as the best SS at the time). His next goal was to be the all-time HR king. As I mentioned previously, he began to "bulk up" in Tex. In fact, it could be argued that he left Seattle, in part, because he didn't like hitting at Safeco. Despite putting up insane numbers in Texas, he wasn't getting the attention he felt he deserved. That would only come in the northeast. Deal to Boston was killed by the union, the Mets had already declined on his services during FA. I guess that left Philly and NYY. I don't know if Philly was interested but the biggest stage is NYY. He had to move to 3B to make that happen. He did, voluntarily. IF I believed he was/is a team-first guy, I'd give him credit for that. But I don't think that's the case.
So, to me, he chose to become a 3B in order to increase his chances of becoming all-time leader in HR and increase his recognition within the game of baseball legends.
And, yes, I do find him to be a primadonna. Had he not announced opting out of his contract DURING the WS, I'd have probably looked at him in a different manner. But still as a 3B.