That everything is fated, and all that happens is just as it is meant to be. That we are simply actors playing out a script, or self-aware sacks of flesh along for the roller coaster ride that is life?
I think it takes an enormous level of acceptance of life and its horrors to believe that all that happens, happens for the best. To give up control and submit to fate. Or maybe it is fear that drives this belief: the fear of uncertainty.
Because when we were children, our days and our futures were planned out for us. A ten year old knows that he'll be in sixth grade next year, and that he will get that bicycle for his birthday. He knows that he will start swimming lessons this summer and that his parents will take him to his grandparents for the holidays. There are no uncertainties, no decisions to be made. Life is so easy.
Growing up is a disillusioning process of discovering that no one really knows what is going to happen tomorrow. In my opinion, giving into destiny is a reversion to the childhood acceptance of a faceless parent-figure who knows what's best for you and will take care of everything.
Because belief in karma gives you complete control: if you get sick it is your fault; if you get rich, you worked hard and deserved it; if you get injured in an accident, you must have done something to deserve that too.
It's a simplistic view centered on oneself that takes away agency from other creatures and the impact that chance events, and your environment, have on you. I wonder how you can have empathy for someone else if you believe in karma. After all, if they don't have it as good as you, it's only because you deserve it more.
I myself have been accused of this belief in karma. I don't necessarily think that if I do the right thing, that the world will pay me back. I just think that society is more than the sum of its individual elements. That not only does your environment impact you, but that you can influence it back. That you aren't a slave to fate but one of its makers.
As adults, we all find a way to cope with reality. Some with destiny (acceptance), and some with karma (control). And then there are those who choose neither, but instead try and analyze and root cause everything to figure out why things are not the way they should be. They are the ones who have no time to discuss philosophy. They are the ones who change the world.