The first turning down was probably a smart career move, as there might be better opportunities for making your mark as the XO of a big starship than as the CO of a small one.
The later ones... Well, Riker turned down the Melbourne at a time he was the king of the Starfleet hill, and nothing, bar nothing, could have reflected negatively on his career. The Aries thing is probably the most significant one here - and it seems related to Riker sorting out his father issues. He's finally left his biological father behind him, and accepted Picard as his adoptive one, so it makes sense to cling on to the good thing and stick with Picard, even if this harms Riker's career.
Without the father issues, any officer would probably choose differently at that particular junction.
Reluctant as I am to agree with you, this makes quite a bit of sense to me. Riker sees Picard as a mentor, and a guide to the adult world of Starship Captaincy after the adolescence of first-officer-hood.
And it certainly adds depth to the way Riker reacts to Picard's apparent death in Gambit.