Why are exchanges freezing accounts?
Probably because they have observed suspicious activity/activities. At least that's what they say. And then you would suddenly wonder why your own monotonous account itself was frozen when nothing unusual happened to it for years. And so I would say they sometimes, or perhaps regularly, freeze accounts for the sake of nothing or probably to require them to undergo KYC.
What can I do to make them not freeze my account?
First and foremost, exchanges are weird animals. Having said that, they could make you not able to not freeze your account. How many users complained of exchanges keeping their accounts locked despite them fully acquiescing to all their demands?
What are the best exchanges to use?
I like no kyc exchanges, please do not advice me to use any kyc exchange. I prefer exchanges that do not demand for kyc or the ones that makes kyc not mandatory?
I'd say it is generally safer to use top exchanges. But it is never a guarantee that their best is good enough.
KYC/AML is legally required from exchanges but many are providing a certain limit to non-KYC accounts. Binance, KuCoin, BitFinex, LiveCoin, and so on are providing their services even to non-verified accounts.
But, again, be very careful as your account may suddenly hit an unknown trigger and, despite staying on your non-verified limit, KYC may be required out of the blue. So you better brace yourself for that.