Do most PhD students want to quit grad school at some point, or is this a thought that only the most miserable students entertain?
If at any time you feel
You are not understanding what supervisor is saying.
You are not on the right path.
Your colleagues are performing better than you.
Only you don't understand a thing and lagging behind in deadlines.
Your supervisor is not happy with your work.
Your advisor is rude to you but you saw him laughing with another student.
Your work seems easy but others' work is very impressive.
You just want to quit.
You can't quit because of social pressure.
You just wanna go with flow.
Any other symptoms like above mean you are doing PhD and almost every student get such mood swings any time. I almost gone through all but when I talked with my colleagues they were thinking exactly like me, some even saying my work is very nice and their work is easy etc... So don't worry just work and read.