Why do some men don't realise how lucky they are to be loved?

So. I am a mid 20s man. I am single and accepting that despite being rich and fit and polite I am not women's type so I will probably never get into a real relationship. I wonder how there are men out there who are in relationships and treat their partner badly. Sometimes men get girlfriends/spouses that are way above their level but they can't appreciate it. Why? Other men like me on the rare occasion they go on a date are over the moon yet some people have a perfect girlfriend and don't appreciate it.

I know this is nice guy mindset but I am still confused. It's like winning a million from the lottery and then complaining that you did not win 5. Why would you do that?