Lately, I've been writing lots about strategies to finish a PhD in time, successfully and without too much stress.
I finished mine, and I'm glad I did (and will write a post with reasons NOT to quit the PhD soon). But sometimes it may be a better deal to quit and that is a valid decision, too.
Here is a completely subjective selection of reasons to quit before graduation:
You realize you don't like research.
Maybe you're passionate about your topic but you don't want to research it. Maybe you prefer to approach it as a writer, activist or entrepreneur.
Maybe your topic is not as exciting as you thought.
You thought your project would keep you interested for 3, 4 or more years but it doesn't. Maybe you enjoy being a researcher but would prefer working on something else.
It doesn't make you feel good.
Bad work conditions, excessive competition, poor work-life balance, etc. etc. Academia can be a harsh environment and if it is hurting you in any way that is a great reason to quit.
If you consider quitting your PhD, ask yourself:
Are any of these reasons true for you?
Is your feeling temporary or is it a long-term thing?
When you think ahead ten years or so, will quitting (or staying) have a net positive effect on your life?
Quitting a big project like a PhD is a decision that shouldn't be taken lightly. Maybe these reflections can help you find out what's right for you.