All happy PhDs are alike. Every unhappy PhD is unhappy in its own way.
There was a story in the Guardian not long ago about a young scientist who had supposedly found a dream PhD position, only to then find the lab ill-equipped, the supervisor under-prepared and the institution unsupportive.
Now, my own PhD was a happy experience (not just saying that because my supervisor is probably reading this. Hi Nicolas! But it really was.) But I have seen so many unhappy PhDs, and so many different ways in which a PhD can fail.