Why? Random genetic mutation, probably.
Although our genome has been know to incorporate virus DNA. Still an accident.
But it either benefits survival in some way or does not harm it or we wouldn't be here. For now.
Basically, this is one of the chemicals that gives us pleasure. Our bodies produce it on it's own. One of the biochemical processes that make us human - to love and hurt and cherish and hate.
That there are so many plants and fungi that match this receptor is what I find interesting. Especially their role in our spiritual evolution.
In the jargon of tech: "It's not a bug; it's a feature!"