Annie and Cole go to a pub. Cole pays the bill; Annie says "I'll get the next one."
If Annie has a sense of fairness, then she will reciprocally pay the bill next time.
As it happens, Annie doesn't have any such sense.
Her room gets cold at night; he lends her a space heater.
She lost her headphones; he lends her his spares.
The weather turns very cold; he lends her his gloves.
She wants to put a nail in the wall; he buys a hammer.
The list goes on.
Annie comes home one evening with groceries. Cole is a little short on food this day and a little hungry.
"Can I have one of your fish cakes?" asks Cole.
"What, you think I owe you something?" Annie snaps.
"But, well, I made you dinner last night, and paid the bill at the pub-"
"Oh fine, have the bloody fish cake. God." She leaves, shutting the door hard behind her.
...
"I think I'll just go to sleep."
This is not a reciprocal relationship, materially speaking. It's more like an altruistic one.