You are not a machine.
If you were, then it would be fine if you constrained yourself to strictly logical thoughts.
But, being a living creature programmed to not self-terminate, you’re stuck here for awhile, and you really can make the best of things by carefully choosing what you do and don’t think about.
Even taking into account things we should think about, for ourselves and for society, we still have a lot of remaining items for which thinking is optional.
Don’t think in ways that will make you unhappy, if you have a choice, and, in particular, avoid cynicism.
Attitude is everything. You can choose to have a great life, or a terrible life, because, in most cases, if you have the basics of life(food, shelter, safety, love) it’s not an objective calculation; it’s entirely how you view it.