Most people work because they can exchange said work for money, which allows them to keep a roof above their head and buy food.
So basically, survival. It is not something you do because you want to, but something you need to do in order not to perish and die.
I'll concede some jobs and fields are more rewarding to others, and if you find a workplace to which your values, goals and ambitions somewhat align, it can be, at the very least, tolerable.
But ultimately (and this might be influenced by that other thread about humans going extinct in the next hundred years), I think people despise work because the whole system is based on greed that rewards a very selected few.
About a 100 years ago, a very greedy person decided that it would be a good idea for people to work 8 hours per day, 5 days of the week, in a row, and for some reason, we're still doing that, a 100 years later.
Or, most of the world still is, at least.
On a similar note (greed), the ultimate goal of most (if not all) companies is to generate revenue. To sell. To make money.
People who place a lot of interest in money usually enjoy work. People with "greater" ambitions eventually burn out because they feel a disconnect between their individual goals and beliefs and those of their employer (to make money).
I think most jobs consist of creating solutions to man-made problems, so it is an infinite loop of meaninglessness that ultimately doesn't amount to nothing but numbers (virtual currency, man-made values at this point).
For example, even in the gaming industry, with all of its glory and motivated professionals, how would it be possible for someone whose job consists of creating mechanisms to get people to spend as much money as possible on a gacha game (which is basically gambling and can hardly be categorized as a work of art, as opposed to perhaps other games with higher ambitions) to not despise his job when said job turned the field that person cherished the most into a device made to pray on human weakness and generate numbers on a bank account somewhere?
How does that person keep themselves motivated while knowing they won't make a positive difference in the world, that history won't remember them, and that, ultimately, all of their existence will remain meaningless? Especially when they'd rather use their limited time on this Earth to do something more fulfilling but are prohibited from doing so because of their need to work everyday?
It wouldn't be that bad if people were allowed to just exchange their time for money, using the money they earn to enjoy their life outside work.
But most companies will demand not only your time, but also, for some reason, your loyalty and emotional commitment as well. ("
emotional labor")
You have to get to and from work everyday. You need to go to bed early so that you can perform adequately the next day, etc. Suddenly, your entire life revolves about that one aspect of your life.
Furthermore, your also have to worry about reaching long term goals arbitrarily set by your employer if you don't want a decrease in your livelihood, which becomes critical if you need to provide for people other than yourself.
Basically, the current system allows your employer to govern almost every aspect of your life, not just those 8 hours every day.
Suddenly, what started as "working to live" becomes "living to work".
So, it is hard to not despise work when you are forced to live within that very contradiction for your entire adult life, unless you win the lottery or something, for the sake of others.
TL;DR: Nihilism. Excessively long hours, unregulated greed and ultimate lack of meaning.