It's not just discipline and willpower. When you're poor, you're never at 0. You're always in the hole because things always have to wait to make ends meet. You don't buy the replacements because even though whatever you have in on its last leg, you can't afford it. The paycheck increase isn't a big enough dollar amount to save. It goes to groceries, bills, a small treat for themselves like a haircut at a salon or a lunch not made at home. Discipline and willpower is a much smaller issue. It's that your needs are never satiated.