system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why does being alone feel bad? What chemical in our brain stops flowing when we don't socialize sufficiently?
gpt
Oxytocin is part of it.

Oxytocin (not to be confused with oxycodone the narcotic) is a hormone responsible for social bonding that is released when you touch other people. Cuddling and hand holding or other types of touch (yes, definitely sex) cause your body to release oxytocin which makes you feel good. 

Oxytocin has been shown to help with anxiety, depression etc as well as encouraging monogamy in people who are in relationships.

Edit: in addition to human and human contact, oxycotin can also being produced when you interact with your pets. Go therapy dogs!