Motivations are open to interpretation, as is any career or specialization. I think its important to remember that. As GM, your interpretation may not synch with that of the player. Also, people change. What motivated a PC right out of the gate may get pushed aside or even abandoned by something in the player's real life or something interesting and engaging within the game.
As an example, I played a barbarian long ago and had written a rather in-depth backstory, only to have the character forget literally everything about his past; we'd found a bowl stone filled with clear "water" in a wooded glade, we needed fresh water, so I tasted it to make sure it was okay, but it turned out to be a dryad's forget potion, something to make a traveler stay in the glade. And just like that my entire backstory became irrelevant.
The same can happen to anyone, player and PC, for any reason, at any time.