I, for one, like new things if, and only if, they are an improvement over the old things. That's why I use a computer in the first place: to improve my life and make things easier. Anything that gets in the way of that gets the vitriol poured on.
For something as fundamental as the UI, I have a substantial investment in the old way of doing things. Throwing that away means I have to start learning again, and it'll take a while to get up to the same speed I had with the old UI. This is all wasted time, so the new UI has to be a lot better than the old one to make a switch worthwhile.
Many of the UI changes in Windows have not been improvements at all. Instead MS seemingly randomly moved things around (Control Panel), or they removed functionality (Start screen). So yes, we consider this change for change's sake. I, for one, would welcome some actual improvements instead.
There's also the alienation factor. A new UI feels as if people have broken into my home and rearranged things.