My Swiss friend saw Vietnam, was once enough?

We met while I was studying English online many years ago.

I asked what his hobby was. He said he loved traveling. I asked his if he'd ever come to Vietnam? He answered "No."

I then tried my best to promote Vietnam's scenery, food and culture to persuade him to visit the country.

In the summer six years ago, he came to visit.

But he has never returned: he always goes to Thailand instead.

He still praises Vietnam and its people as "beautiful, friendly and hospitable," and he has continued to promise to return again. But it has not happened yet.

This makes me wonder: why hasn't he come back?

Why do you think that tourists who see Vietnam once often do not return for a second visit?