What's Happening in the Brain
Participating with other people in leisure activities requires a lot of things in the brain to be working properly. In the first instance, the person has to be able to remember the activity, and it needs to make sense to them. As part of that, they need to conceive of and project that activity as a future event which involves their taking part in it. They need to be somewhat confident that they can still do the activity, that something good will come of it, and that nothing bad will happen. When leisure activities involve other people, they also need to be able to be aware of those people, and their motivations, and trust that they can interact with that person. Finally, they need to be able to perform the activity, and to monitor their own performance. These last several functions are typically associated with so-called 'executive function', and involve the very front parts of the brain, known as the 'pre-frontal cortex '.
In short, leisure activities require not just the ability to perform them (i.e., hobbies require so-called 'procedural memory '), but also judgment, initiative, memory and language. Given that so much brain circuitry must be activated, it is not surprising that for many people, loss of leisure activities is an early sign of dementia.
Even though the loss of leisure activities is an early sign of dementia, many activities can seem to be particularly 'over-learned'. In consequence, we sometimes encounter people with even moderate dementia who are still able to play games such as bridge or cribbage.