Cooperative activities trigger oxytocin release, the same "bonding hormone" associated with physical touch and eye contact. When you high-five after a raid or groan together at a game over screen, your brain is literally coding your partner as "teammate."
Additionally, the concept of "flow state" (being fully absorbed in an activity) is amplified in co-op gaming. Couples in shared flow states report feeling more connected, more in-sync, and more satisfied with their relationship. It's the same mechanism behind why couples who dance, cook, or exercise together feel closer.
The difference? Gaming is more accessible, more varied, and doesn't require leaving the house. For introverts, long-distance couples, or people with social anxiety, it's uniquely powerful.