Stress comes from many different sources, but one particularly dangerous form of stress is loneliness. Being cut off from other people takes its toll, and can have effects right down to a cellular level, as described in this article on Vox.
When I was researching the lifestyle factors that affect cognition, this came up a lot. Having a strong social network is important for maintaining healthy cognition, today and into old age. Not only can a community provide a buffer against stress (which is known to affect the brain directly), but maintaining relationships is a cognitively demanding task; as we all know, people are complicated!
It's a reminder that optimizing your cognition isn't all about brain games. Calling up an old friend may have just as much impact.