Why do so many people hate themselves?
There are many reasons. I can only speak for myself. My personality type according to the Myer Briggs classification is an ENFP - T. Robin Williams is an ENFP. As part of my personality I always want to be a a better me and I tend to take criticism personally. So that is one aspect.
Your childhood seems to have a huge influence on your mental health, self esteem and whether you like yourself. Although my father and mother loved me, and I had a pretty awesome childhood, I did experience rejection repeatedly, especially in school. That wound has had a significant impact on how I treat myself.
In my case I generally resort to people pleasing tactics to be liked and accepted and I am very hard on myself when I make mistakes. I also tend to neglect to look after myself.
I was also sexually abused once, by a a boy two years older than me, but also still a child himself. That has among other things affected my boundaries negatively, and has presented as putting my needs after everyone else's needs, once again looking like I dislike myself.
This dislike of yourself that you observed has very far reaching implications.
I decided to find a job that is very difficult. My plan was to become this leader in the community so that people would like me and accept me. I wanted to be loved. I did not know that you do not need to perform to be loved, and that that is not love at all. I thought to be loved, you need to deserve it!
So... I became a vet. Sure I loved animals. Sure I am not a bad vet, but I have never been completely happy.
Fast forward 15 plus years into the vet industry and thanks to a loving husband among other things, the light bulb goes on that he loves me just as I am, just for being me. Unconditional love... Suddenly my main motivation for being a vet is gone. With that gone, the stress and trauma of a sensitive person like me having to do euthenasias is suddenly not worth the personal price I pay.
Now that I have grown to this point, I need to be brave. I need to like myself enough to say: you owe it to yourself to make a career change. To feel passionate about something again. To feel joy and peace again. I am terrified. But being brave doesn't mean that you are not scared.
Comparing yourself to others also tend to be very toxic. Either way you set yourself up to be in a bad place. If you conclude that you are better off than the other person, you may become arrogant. If you find yourself lacking you may think badly about yourself. Take it from me, there will always be someone more clever than you, faster than you, richer than you.
So to everyone disliking themselves I want to say : You are the best you, spoil yourself a bit, love yourself because you are one of a kind.