Both are right. Concern for one's own or a spouse's health is the most common reason seniors give for making a move that might otherwise be postponed indefinitely.
Loneliness motivates others, especially if they can no longer drive.
One way to handle the downside of moving is to cast the new home in a favorable light. As people age, it is natural for them to feel more vulnerable, so housing designed for safety and access can provide a psychological lift. Proximity to medical, social, and support services can be a welcome change. For those who live alone, moving to a community of peers may offer the promise of a new circle of friends. Making the decision to move is the first, but not necessarily the hardest, step.