Feeling obligated to return a favor, share resources, trade an item, etc.
"Only minimally, with a very strict and limited donation feature that is also exclusive to one in-game type of currency."
"Cultures specialize in specific resource productions; you need all the resources to progress, so you must trade for the resources you don't have."
"You can 'lend' a unit for their battles. This doesn't give them the unit."