I've thought about an extension to this idea that reciprocity is not just a side factor in relationships, but that relationships often build on top of reciprocity. Let's walk through a hypothetical romantic relationship from beginning middle to end from a reciprocity framework. I believe it applies to friendships and other relationships, but I think results are amplified in romantic relationships.
You try to get a stranger's number or try to buy her a drink. In both cases, you could be violating social reciprocity. In one case, you're asking for too much and in the other, you're giving too much. Relationships build off of gradual increase in reciprocity, so maybe first you strike up a conversation, then buy a drink, then get coffee, then get dinner, etc. It's weird for someone to "try too hard." And I think it's why there is a 'rule' for a guy to wait 3 days after getting a number to contact a girl, because you don't want to 'seem too desperate' and break the social rules of reciprocity.
During the relationship, reciprocity is even more important. If one person is giving too much relative to the other person, the relationship will quickly end. There's a lot of factors that would sway the definition of "giving too much" such as perception of giving/receiving, resilience of each individual, opportunity cost, etc. For example, maybe a guy is giving what he thinks is a large amount of his time, and the girl thinks he's not giving enough. Maybe she's flirting with other guys, but he is in love and cannot end the relationship. But my main point is that when things FEEL equal, everything seems right and when things FEEL unequal, the relationship becomes shaky. When both parties think they're spending an appropriate amount of time together, the relationship is great. When she wants him to meet her parents and he doesn't think that's a good idea, the relationship gets shaky. Reciprocity.
In friendships, there are far fewer expectations and the amount of giving is usually low. The results aren't as amplified, but even all the "20 things to do in your 20's" links all over the internet will tell you to cut off friends who are leeches, aka when the relationship is one-way. Great friendships are ones where you give and receive the most: the ones where you share your deepest feelings or when you make a sacrifice in the other's time of need. When a friend asks something big of you, its likely to be a turning point in the relationship, for much better or much worse.