What's great about Walt's backstory is that it's anchored by the same thing as his "cooking": the desire for the quick fix. He stepped out of his passion (with both Grey Matter AND Ms. Black) for the guaranteed, stable paycheck of teaching rather than risking failure for the greater success of his startup. With the cancer diagnosis, he's jolted into a) not giving a fuck and b) knowing that his decision to go into teaching has left his family fiscally fucked. It's why he initially plans to forgo treatment. His pride, which is what will lead him deeper and deeper into the meth game, plus whatever residual longing he has for Laurie, keeps him from accepting the payout from the Blacks. And so, when he's presented with another "quick fix," he takes it. And yes, he could have easily quit once he "got ahead," pride's a motherfucker.