I, too, have wondered how Keith didn't get further in medicine and was still living with Grandma. My guess: he probably spent years wasting his life and/or trying to make sense of his dad and why he didn't really have one. That situation is toxic, and getting that Dialysis Tech job was probably a major achievement for him, regardless of the salary and the hours.
Keith does have goals, however, and with a woman like Krystine by his side (if she stays) he could achieve reasonable ones.
Wanting to be a Dr. probably isn't reasonable, especially since he wants kids... Doctors end up in debt up to their ears before they even have an M.D.!! ...But there are paths in pretty much every big city that make it possible for hard-working students to get nursing degrees, and even advanced nursing degrees, while working a full-time job.
It just isn't easy.