The Vikings believed that sometimes, their gods would visit people in human form - and that in being disrespectful to strangers, you could be disrespecting the gods
Incredibly there was a similar belief in ancient Greece. They believed that sometimes the gods, particularly Zeus, dressed in human form, and acted as a foreigner or a beggar to test people's hospitality and virtue. It is said that once, Zeus and Hermes went door to door through a city in Greece, disguised as peasants, asking people for a place to spend the night, but no one attended them and only two people, an elderly couple named Philemon and Baucis, opened them the doors. The couple soon realized that their guests were gods, and the latter told them that they were going to destroy the city due to the lack of virtue and hospitality that was there, allowing these two people to escape while they sent a flood to the city. Myths like these were so widespread throughout Greece that even in Christianity it is alluded to when in the Book of Acts it is said that when the apostles Paul and Barnabas visited Lystra (Greek city), they were mistaken for gods (Hermes and Zeus respectively) for doing apparent miracles.
Zeus, the most important god in Greek mythology, was the patron of hospitality, and for that reason they gave him the epithet of Xenios, which means something like "foreigner", from which words like xenophobia come, which means fear of foreigners. It is incredible that many times they want to sell themselves to the western tradition as xenophobic, when in reality, and in its origins, it's quite the opposite.
Although yes, I had already heard about the nine virtues, although I really don't know anything in depth, not much about the Vikings either, so, interesting to read.