Cubs Den writes on the maturation of Starlin Castro. That includes both his approach at the plate and the way he acts on the field and with his teammates:

Renteria’s words are revealing. The words “perceived deficits” hints that there are things we have gotten wrong about Castro over the past couple of years, yet a “much better” teammate indicates there was at least some truth behind the concerns. While we can’t know for certain what kind of teammate Castro was, we can at least imply that it was less than ideal.

Rick Renteria likely deserves some credit here. The approach with Castro in the past was to criticize publicly his lack of hustle and mental mistakes in the field. Maybe Castro changed on his own, but maybe Renteria figured out the right buttons to push.