Even if you're aware of it and consciously work against it, tastes change as your salary increases. If it really was simple to keep your costs fixed and just save an increasingly-larger portion of your take-home, there would be a lot more rich people and many more early retirees.

For example, let's say you're living in the hood, out by Mountain View High School in Orem (joking - but consider a typical home in the Provo/Orem area). Suddenly, your boss increases your salary to $2m pre-tax and gives you assurances that he will pay you that much money each year for the next 4 years - even if he later decides to fire you. Over the course of the next 2-3 years, would you really just sock all that extra cash away? Most of us would quickly think something like "Sure would be nice to get a bigger/nicer/newer/better located house." And next thing you know, you're buying a house in Alpine, or building a house on land in Spanish Fork. And you'll rationalize it by saying something like "House prices always go up in Utah County anyway, so the money isn't 'gone,' it's just changed form" - but there goes all of next year's take-home.

Most people would also feel like they need to look the part, and they usually rationalize that to mean buying nice versions of whatever they like (cars, clothes, watches, guns, RVs, ATVs or all of the above). And you can't just treat yourself - you love your spouse and they've been at your side for all of this... And so they get nice stuff, too (turns out that a nice handbag costs almost as much as a nice wristwatch; her optioned-out Yukon costs as much as your new Audi).

All of that is super-easy to fall into, even if you know it's coming and do your best to resist.