Simple answer – lifestyle creep. Over the years as you get older and keep moving up in your career you get accustomed to increasingly finer things in life. It's perfectly normal to drive a beater in your 20s, but you don't want to take your kids to soccer practice in one. It's normal to couch surf with friends when traveling in your 20s, but not when you are on a family vacation to Disneyland. Add in a hundred more of these and suddenly that $40K/yr that seemed like a king's ransom a couple decades ago isn't quite enough to make ends meet.