I don't want my coworkers to know how much I make because if I'm making substantially more or less than them, it will affect how they perceive and interact with me.
This isn't really an issue, as far as I can tell. I feel in general most people get paid fairly, so if you're doing the same work as someone else the pay won't be that different.
I've always shared and talked salary with my coworkers, both when I was earning the least and the most of my coworkers. When I earned the least, they helped me with how to approach my boss for a raise; and now when I'm the best paid, I help my coworkers get what they deserve.
Or assume you do 2x better work, they initially pay you the same, then you get a better offer elsewhere, and they would want to match, but if they do, you would be getting paid a lot more than your coworkers.
(Note though: I don't think many people acyually check each others tax returns, as that kind of already implies enviousness.)
That's a strange assumption to make. Of course I want everyone to be treated fairly.
> why should I be more loyal to some rich owner
It's not a matter of loyalty to some rich owner (particularly if I'm making much less than my coworkers[1]). It's a matter of getting along in the workplace.
[1] I have, and probably will again, taken positions where I was overtly underpaid because I was getting something else out of the job that I valued more than the money. But (at least in the US), people tend to equate your pay level with your skill level and value, so I certainly wouldn't want my coworkers to know.
I wouldn't directly check it, if they see that I have done so, but if I could have someone else check or similar, I would definitely get it checked what everyone makes. Not specifically because of envy, but it would just be a no brainer thing to do in terms of understanding the market.