I disagree, if someone draws motivation and satisfaction from Epictetus' quote then that's great and I don't want to cast any shade on it. But I think it's very dated and we have better ideas today.

Just to put forward a few recent criticisms:

#1.

This quote is a very literal example of a sort of "man up" style of prescriptive thinking which tells you to solve your behavioral problems by Being More Manly. For the vast majority of people, being told to Be A Man doesn't work to alter behavior in the long run, it discourages self esteem and offers no practical guidance. If studying mammalian behavior is any guide, this sort of advice is likely a product of a man's instinctual dominance drive. Males love to give advice because when it sounds good it helps them accrue status and cement their position in the dominance hierarchy, which increases their mating options.

We have numerous moderns alternative to Be A Man if you want to change your behaviors, the best ones are based on the past 50 years of research into how the brain works. They typically focus on understanding emotional, unconscious, or instinctual behavior and working out how to process these things more productively. Examples include cognitive behavioral therapy (and a host of similar emerging therapies), or BJ Foggs' Tiny Habits framework (which was recently shared on HN).

#2.

For a criticism of a totally different tack, Epictetus and Seneca are both operating within a rigid framework that privileges values such as efficiency, self-improvement, and accomplishment. Presumably, they see these things as the best path to happiness.

Why are these values privileged? What authority has established that any of them, including personal happiness, are important and meaningful to pursue? In the past century we've been exposed to multiple strains of existentialist, postmodernist, and nihilist thought which all emerge from the same revelation--existence fundamentally has no meaning, all value systems are human constructs, and the universe is much larger than our day to day struggles.

All our traditional values are now beset by refutations. We believed in God, but centuries of scientific progress have dispelled the evidence supporting His existence. We believed in civilization, but our largest societies went on to slaughter millions. We thought the universe revolved around us, but learned that we're ants crawling on the face of a pale blue dot, of no consequence in the cosmos to any but our own self-centered perspective.

This might all sound pretty dark, but the good news is that nothing you do is really important. The universe will remain unperturbed no matter what actions you take (or don't)! So if you're stressing about how well you live, remember that you're merely chasing fictions in a framework that someone else cooked up.

This is a modern day revelation that would never have been achieved without modern science and rationalism, and is still developing. There are as many views on how to process existence's meaninglessness as there are philosophers. To name a few: Nietschze's will to power, Camus' absurdism, Sartre's pursuit of personal freedom. Alternatively it's fine to spend your afternoon playing Xbox, no matter what your parents say.

#3,4,5++

I'm sure there are more!