We all have comfort zones. Some employees must remain within theirs to thrive in the workplace; others thrive only when they get to step out of theirs now and then. You can't manage without both types on your team.
Those who thrive because their tasks remain within their comfort zone are your loyalists, your Dwellers. They will stay happy (and stay forever) as long as you don't force them out of their zone. Not everyone is happy advancing into new territory. Make Dwellers uncomfortable with new challenges and you make them temporary. Just don't. You need them.
Others are bored when kept within their zone too long without a challenge. They are your Expanders. They will love their job as long as you help them expand the limits of their comfort zones. Not everyone is happy with a predictable routine. Deprive Expanders of new tasks or responsibilities and they will seek new employment that invites them beyond their perimeter. Don't deprive them. You need them, too.
Empowered Healthcare Managers know into which category every member of their staff falls, and don't force Dwellers to be Expanders or vice versa.