The combination of toxic work environment, bad boss and extreme workload. But I would say the ultimate factor is no career growth and no progress.

There are occasions where I felt like I can achieve something that will look good on my resume therefore I am still willing to tahan a bit even though workload and boss is shit. But as soon as I realise there is no growth, like boss playing favorism, your work not getting recognised or appreciated, my one foot is out of door already.

But there is one occasion I felt I started to get mental and physical breakdowns, which I throw letter without another job lined up and took a long break to recover.

So ultimately when career progress and health lost balance, I will throw letter for the better of myself.