Why do artists hate their own work ??

Because we see the flaws and compare our work to other artwork we find superior.

I suspect it's because all art captures (essentially) a moment.
The finished work captures the moment of finishing that particular concept.
And having completed it, the artist feels accomplished.

Now, if you were to immediately sell/give it away, your mind would be clear.
Job done mission accomplished lets raid the fridge !

But the fact that you still retain possession of it means you'll look at it frequently and think.....yeah that image may well have been brilliant THEN, but NOW looking at it I think I'd do THAT differently and THIS in a different colour......and so doubt creeps in and ultimately you end up hating your creation !

I'm pretty sure writers; performers; songwriters; composers etc. ALL feel the same at reflection of their work.

I think they always see room for improvement in their own work

Look at this another artist made a small encouraging note for us all: https://mobile.twitter.com/avatarkasia/status/960114670790152193?s=19

Probably because artists know how hard it is to make a work of art, and they know their own limitations.

Because it’s never perfect

I'm always harder on myself than I am on others.

I don't hate mine, but I'm very critical of it until enough time has passed. You always think about ways it can be improved still.

Hate is pretty strong a word. Intensely critical? That's closer, and the only way we improve.

maybe it sucks or maybe it's just gay to love your own gay work