What is 'fair' is always debatable, but ... let me tell you one way I look at it.
I see democratic governments as an extension of society; and their most important service is a reasonably stable, safe society (with a few exceptions, there's no rioting; I can walk around the streets without fear of getting killed; I don't even worry about whether I lock my hour or car doors).
Given a live a nice upper/middle class life, I value this a lot, and am willing to pay high taxes for it; if I'm a poor teenager in the ghetto, I'm not getting as much from this; Bill Gates is getting much more than I am from this; it makes sense he pays a lot more than me, and that I pay a lot more than the teen from the ghetto.
Imagine society goes to heck ; do you think Bill (or me :) would keep his money ? his nice life ? his life ? :) If that happens, most people would be worse off than now, but Bill would be much worse off, whereas the teen from the ghetto may only be slightly worse off (and some people might even be better off). It seems 'fair' Bill pays more; it actually seems to me it is smart for Bill to pay more to keep our nice society (which is one reason he, and many other billionaires advocate for higher taxes).