Riker's reticence about taking the Big Chair could have had any number of deep-rooted psychological reasons. Problem is, it ran completely contrary to the narrative arc he followed in "Best of Both Worlds".
The first part of the episode focuses on Riker's failure to move on, including encounters with Shelby (who wants his job and shames him for not leaving it), Picard (who gives him a hard kick in the rear end for his own sake), and Troi (who acknowledges that he has gained much from his time on the Enterprise, then urges him to decide what he wants). In other words, the conflict is set up, but we're not pointed solidly at any resolution just yet.
However, when Picard is taken, Riker has to step up and take command. And yes, some lessons have to be taught, courtesy of Troi and Guinan. But when it's over, Riker has convincingly demonstrated that everything Picard said of him was true (he was ready to work without a net), and everything Shelby said about him is false (he didn't just know how to "play it safe", and he could make the big decisions). Logically, the conclusion of the narrative, as it was played out, is that Riker is ready to step out of Picard's shadow and begin forging his own legend.
It just never happens.
