When Did Rest Become Something We Have to Earn?

I've noticed how uncomfortable people can feel when they're not being productive. Even our free time somehow becomes another opportunity to improve ourselves, learn something, or get ahead. We say we want more time to enjoy life, but when we finally have a quiet afternoon, we sometimes feel guilty for doing absolutely nothing. It makes me wonder when simply enjoying our lives stopped feeling like a good enough use of our time. Have we become so focused on making something of ourselves that we've forgotten how to just exist?