Usually it's jelousy or insecurity, or they are not themselves where they want to be in life so it comes natural to hate others who are.
Take this for example, say you have a group of 5 friends and you all eat a trash diet and you're all obese. What happens when 1 guy decides to start taking action in his life? he cleans up his diet, goes to gym starts becomming confident. The other 4 are going to hate because why do they want to be reminded that they are a piece of
****, it's too painful to have it rubbed in their faces...."Just stay fat bro! why do you want lose all that weight, why get jacked you are fine the way you are stop being so insecure man". That all translates to,
**** you are stunting on my lazy ass and now I feel pain because I know I should be doing it with you but I want to go back to rationalising being fat and eating McDonalds because it's far easier and gives me my dirty little high of good emotions.
Nobody wants to eat a salad, does that give you that little dirty boost? nah dude slam down some more sugar and mcdonalds! I get to feel good right now screw putting in work over the long term even though it pays off 100000x more than eating that big mac. Google hyperbolic discounting. "Working hard pays off later, but procrastination pays off now"
When other people see others doing well they are faced with 3 choices.
1: They either accept that person is doing well and give a nod of approval and carry on with their life doing nothing or perhaps they don't care maybe they are not motivated by the same driving forces.
2: They accept that person is doing well, look at why and think how can I change my own life to become more successful (This doesn't mean becoming super rich It can take many forms of just things you want to do / accomplish) and be doing what I really want to do? this requires incredible willpower, which most people lack.
3: They hate, it's far easier to make nasty comments than it is to take full responsibility of your life. People then try to rationalise their mediocre existence and then say well you know... I just wasn't born with money so he's just lucky! (Not saying money / being rich is the be all end all but I'm saying perhaps the rich kid is doing something he's really passionate about because he was fortunate enough to have funding) so i'm going to sit here and do nothing and be bitter and write nasty comments on youtube. Little do they realise that attitude is exactly what pushes them further down. It's nothing more than a vicious cycle which ends up with you being depressed.
TL DR. Human psychology
LOLOLOL
Edit: And yes I agree there is unfair
******** happening in the world. Look at sweatshops in china etc where people are treated like dirt for others to profit from. However what I am saying is if you are unfortunate enough to be stuck in such a place complaining not help you, but changing your attitude and taking massive action will, Yes it'll be hard and it'll suck BALLS but what choice do you really have?. I've been there myself....
I personally think that if you have made a lot of money then you have a moral duty to help out those less fortunate, be that by paying the correct amount of tax
This I agree with too, some people have benefited a lot from societies structure and I think yes you may have worked hard, but without all your employees your company would be non existent. I think there is a balance to be struck. Those higher up in the wealth ladder should bear a little more responsibility than those at the bottom working for minimum wage.