Yeah, I sometimes think that. My last boyfriend was vile. Started off charming and lovely and thoughtful and I truly thought I'd finally found the One. Just a few weeks in, he came round to make me dinner the night before a hospital appointment I was worried about and on the day of it he took the afternoon off work to pick me up and take me out for a drink and then dinner afterwards. Started talking early on about getting married and living together (I know, I know) My normal guard was lowered because we had so many friends in common and they all vouched for him and I vaguely knew him as an acquaintance.
As soon as we became official bf/gf, he changed almost overnight. Started negging me and insulting me. Whereas we'd previously chatted and chatted for hours, he suddenly started creating these weird uncomfortable silences, and when I tried to talk, he rolled his eyes and said "you've talked about this a million times". He mocked me for being nervous about Covid when the pandemic started and washing my hands when I got in, even though that was literally the government advice at the time, and I have underlying issues including asthma. Just total gaslighting and cruelty, interspersed with occasionally being decent again, to keep me on the hook for sex and hoping the old him would come back. If we had a good couple of weeks with no fights or arguing, he'd invent something to get upset about and make me cry. Once it was the way I was holding a spatula. Everything I did was wrong, everything I said was stupid or pointless.
I know for sure that if I meet someone genuinely kind and decent, I will never, ever take him for granted.