... I'm never sure why people feel so strongly one way or another.
Me too. And I'm a person who feels strongly about many things, but whether to take off or not take off shoes isn't one that tweaks me.
DH's mom is Korean, so they definitely have a shoes off home. All of his friends who come over automatically take off their shoes, as they grew up visiting DH's home and are used to it. I go either way, sometimes leaving my shoes on and sometimes taking them off, just depends on how I'm feeling (at MY home, NOT MIL's home...there I take off my shoes!).
I remember one holiday party that my friend called an open house...I guess she meant that it had extended hours to drop by, but I never really understood it. When we would visit her house normally, shoes would be off. So as we entered, we took off our shoes. She had a half up half down house, so the entryway was no man's land...when we descended to the living room, we noticed that no one, not even the hosts, had shoes off. D'oh! I felt silly.
For anyone in the Tacoma/Olympia/surrounding area in WA, though, you really should be taking off shoes. Thanks to the Asarco smelter, we do have problems in the soil, and it's not just the dirt you have to worry about. So for the person in WA who hates having shoes off, it's possible your friends are just used to the
Soil Safety program, if you or they are from the Tacoma and beyond area, and that's why they take off shoes.