I disagree with this notion (despite abhorring quitting). I think quitting can be empowering and freeing — it’s more important to know when to quit and when to stick with it. The key is being able to differentiate misery and challenge. People shouldn’t quit just because they face a challenge — but if they are spending a majority of their time doing something which begets misery, then they should consider their options.

That being said, if you finish 50–85% of something, then don’t give up, no matter how difficult it is. Make the most of it and follow through. But if you’re 25% of the way there, and 80% sure you hate it, then give it up and move on to something which is a better use of your time.