Someone already answered the part about physical activity so I'll tackle social isolation.

There's multiple factors at play:

1. Humans are social animals and require social bonds. Loneliness is meant to force you to socialize same as hunger is meant to force you to eat, thirst is meant to force you to drink water, and pain is meant to force you to tend to injuries.

2. This evolved because in prehistoric times being shunned by your tribe was more or less a death sentence. If you didn't have anyone looking out for you it was going to be very difficult to survive. Obviously the consequences for social isolation have gotten a lot less extreme modern times, but you're still at a disadvantage. If you have someone living with you you're less likely to die from things like slipping in the shower, having a medical emergency at home, choking on food, etc. You'll also have support if you become unable to take care of yourself.

3. Because of all this, being lonely is inherently stressful. Stress activates your fight or flight response. Chronic stress keeps that system active long term which results in chronic inflammation which results in a lot of bad health outcomes.