Lifestyle creep.

Eating out at the $25 to $50 per person restaurant instead of the $10 to $20 one. Eating out more often, like buying a lunch instead of bringing it.

Buy nicer stuff for the home, especially if you replace it before it wears out, like getting a new couch because you want a new style.

Dropping money on hobbies without really thinking. I enjoy needlework. Sewing, embroidery, etc. You can do this very cheap. Hand sewing or a machine under $200, work with thrifted clothes and fabrics, buy thrifted or estate sale patterns and materials. Or you can spend thousands on sewing machines, buy specialty machines like and embroidery machine or a serger, and stock up on new fabrics that you'd like to use rather than using what you have, and buy materials like wools and organic cottons that are inherently expensive.

Impulse buys. There are always another pair of cute shoes, or a newer piece of tech, or a bigger TV, or a more specific kitchen gadget.

You'll never run out of things to spend money on, so if you aren't making a point of saving money, it'll all disappear amazingly fast. There are plenty of stores of people who won millions in a lottery running through it in just a few years because they treated their 10 million like it was unlimited.