A big part is anxiety about the future. You are not guaranteed economic opportunities in the future, so we make the most we have of the present to ensure we can provide for ourselves in the future.
You also enjoy not subsisting. I enjoy not subsisting.
You don't want to starve if your industry becomes obsolete, you have an accident that leaves you unable to work or some other twist of fate changes your ability to work 10 hours a week at your current level.