I always took it that pre-U.S.S Enterprise, Riker was a young hotshot who was rising through the ranks quickly towards Captaincy. He was the type of guy who prioritized his career (hence all the stuff with Troi, leaving her to take his promotion) and probably had a lot of mental games about how young he could be when he made Admiral, etc.
Then he met Jean-Luc Picard.
I always believed that what held Riker back was Picard, but it was Riker's choice. Picard so embodied the ideals and truths of the Federation, his diplomacy, morality, knowledge, everything, that Riker was absolutely floored. He wanted to study under Picard and be a better human, to promote himself from within rather than meaningless rank. That's what I always believed.