Let's take, "I'm going to eat healthier." Easy enough -- on the next grocery trip you'll cut out the junk food -- it doesn't take much will power to just not buy soda and chips and candy bars. Hell, look at the money you saved!
It's a well documented fact that when you cut out junk food, money literally falls out of your ceiling.
A few hours later, you'll realize that you can't go the whole day without snacking on something. The hunger is distracting you from your work. So, maybe next time you buy some carrots and celery and other shit we think healthy people eat. About two days after that, you can't stand the sight of another vegetable, so you'll find a compromise replacement ... maybe granola bars or something with "just a little sugar" in it. Those are healthy, right? Fucking hippies. And you can't just drink water all day, so maybe an occasional diet soda wouldn't hurt.
In this war between you and your cravings, you'll keep retreating until you find some bit of ground that it doesn't cost you much to keep -- say, cutting out the chips, because it turns out you like pretzels just as much. And just like that, you're "eating healthier," despite the fact that 99.4 percent of your diet is exactly the same and your original resolution was clearly shooting higher. It turns out that movable finish lines are really easy to cross.