I would make the argument that Mary Chase's Harvey argues that sometimes we need to accept the people we love for who they are. It's a simple idea, but in reality, it is incredibly rare that we love people without conditional acceptance.
Mary Chase's play, first introduced in 1944, uses a character named Elwood Down and his sister, Veta Louise, to illustrate the idea that sometimes the people we love are imperfect and that to love them with their imperfections is all right. Elwood is described as being gentlemanly, and more than that, he is a kind character. He talks about what his mother used to say to him as a child—how he needed to be smart and pleasant. He says in the play,
For years I was smart. I recommend pleasant. And you may quote me.
His sister, however, does not see how pleasant he is and instead is blinded by her need to climb societal hierarchy. She misses out on how great of a person he is. Instead, she is focused on the fact that he has an invisible friend, a life-size rabbit. While this is admittedly odd, the question becomes whether this should be his defining characteristic.
Ultimately, Harvey is about whether we should love a person based on our conceptions of their flaws or for who they are in the truest sense.