Ancient Greek hospitality, called xenia, rests on two principles: hosts must offer food, drinks, entertainments, and shelter to travelers, while guests must be respectful, never abuse the hospitality given to them, and reciprocate it by, for example, telling tales of their adventures.
Hospitality used to be sacred, for gods were believed to travel amongst men. Refusing hospitality was offending the gods. Breaking xenia was often the trigger of many myths and stories. Paris taking Helen from his host King Menelaus started the Trojan War, for example.
Even though the custom isn't as common as it used to, we can still witness its influence and consider it common etiquette for tourists and locals alike.
Xenia is a reciprocal relationship where both parties grow better thanks to it.
Xenophobia, the fear of what is foreign, is going against this ancient wisdom: it's everyone's loss, because it dehumanizes us. When I hear about anti-migrant protesters in modern Greece, or anywhere else in Europe for that matter, I can't help but laugh at those so-called patriots forgetting what their Greek ancestors stood for.
Overtourism and other touristic abuses can also be considered as breaking xenia, for it doesn't respect locals.
Travel ethics go both ways. Wherever you go, you have to strive to leave the place and the people within it better than they were before your arrival, and there are many ways to go about it: just share a bit of what makes you you.