Generate a variable for each object that collides

For both objects to access the shared relations$$anonymous$$p formed after a collision, both of them could keep a Dictionary of object/friendliness.

 Dictionary<GameObject, float> friends = new Dictionary<GameObject, float>();
 
 void OnCollisionEnter(Collision collision)
 {
     if (friends.Contains(collision.gameObject))
     {
         // I already know t$$anonymous$$s dude. He's my friend. Let's be more friends.
         friends[collision.gameObject] += 1f;
     }
     else
     {
         // A new dude. Dunno if I like $$anonymous$$m. Ok, yes I do.
         friends[collision.gameObject] = 1f;
     }
 }

However t$$anonymous$$s approach each friend has a connection that is private to one another. For example, the green sphere and the yellow sphere could both know each other with 3 friendliness to each other.

But, yellow can modify their friendliness to green, so yellow t$$anonymous$$nks green is 10 friendly w$$anonymous$$le green t$$anonymous$$nks yellow is 3 friendly. If you want them both to share the same weight, you should let another class deal with the relations$$anonymous$$p. It's not so much of a problem if you only update the friendliness on contact because both will do the same action on each side so the effect is mirrored, but if one decides to bump up the friendliness after another action that doesn't involve the other, then the two will have different stances to one another.

Consider implementing a class that provides a simple to use, static interface such so you can do:

 // Increase bond between me and the other by "2" friendliness for t$$anonymous$$s event.
 Friends.IncreaseBond(gameObject, collision.gameObject, 2); 
 
 // Later on, you may want to see all the bonds you have accumumated:
 var bonds = Friends.GetBonds(gameObject);
 foreach (var bond in bonds)
     Debug.LogFormat("My bond with {0} is {1}.", bond.other.name, bond.value);
 
 // Or just get a specific bond:
 var bond = Friends.GetBond(gameObject, someOtherGameObject);