I don’t know, this whole dating thing today is like buying clothes. You go to the store, try something on, if it looks nice you get it (most people don’t even look inside of the shirt), first time you wear it, most people will say “oh thats nice, where did you get it from”. But if you wear it few times, you wont hear it anymore and you get tired of the same shirt. So you go back to the store and look for something new. Maybe few months later you will look in your closet and say “oh, I didnt wear this shirt for a while, maybe I should wear it again” and thats how we live today in this modern world. Always look for perfection. Always look for something better. 
That’s human nature, humans create their own ideal self, and that ideal self is always changing and is never identical to your actual self. It’s completely natural that humans always look for something better.
Incidentally, I quite like the analogy, it reminds me of how Kant describes marriage as “an arrangement by which two people agree to allow access to each others sexual organs.”
yes, but some people care about the inside of the shirt and the others dont. Its easier to date, its easier to break up, its easier to find someone new. Does it mean that people 100 years ago wasnt happy just because they stayed with the same person until the end?
Maybe, maybe not, they were socialized to do so.
I don’t quite follow. Are you arguing against dating or something along those lines?
That describes some people, not all.