Nietzsche does not believe in truth. There is no 'way the world is,' discoverable by reason' or by any other method. Instead, there are perspectives, ways of viewing the world, or interpretations. And if there is no way the world is, then there is no sense in critiquing ways of understanding the world for not being 'truthful' or matching up with the way the world is. For Nietzsche, your beliefs say more about you then they do about the world. So one can always fruitfully ask whenever someone tells you what they believe: what does this say about them? Nietzsche has deep insight to the darker parts of the human mind, and thus what he finds is not necessarily comfortable or nice. It turns out that many of our prized views are shaped by psychological forces that are not pretty.
Thus, for example, a question about the existence of God turns into a question about the psychology of the believer (or non-believer): what psychological needs make some people believe in such a thing? Kant's adherence to an inflexible moral scheme is not a feature of some moral truth, but rather a feature of Kant's psychology: Why is it that Kant needs morals to be absolute? And Hume's adherence to a flexible moral scheme raises the same kind of question. It, too, is not a feature of some moral truth, but rather flows from Hume's own psychology. It is psychology that shapes beliefs, not the world.
Nietzsche also thinks that physiology shapes psychology. A sickly body is the cause of the sickly mind. We do not choose interpretations or our psychological makeup. It is all determined by our physical well being.
It is also the case that on his view, modern man is man in decline. Weak, effeminate, and above all, sickly. He envisions a war between strong and weak natures, a battle of values, centuries old, roman warrior vs Jewish/Christian, Bird of Prey/Sheep, and the Jewish/Christian value scheme has won. But the scheme is not a choice, the people's value scheme is a product of their psychology and physiology. With the victory of the masses comes the decay of mind and body. Make no mistake about it (and the people who love Nietzsche always get this wrong), the sheep won. There are no more strong birds of prey. We are the sheep. Anyone who thinks he is not is sadly deluded, and more sick than anyone else.
If there is no truth, there still are perspectives that are better than others. On what basis? Psychology. Some perspectives flow from psychological strength and health, others from weakness and sickliness. Nietzsche will then rail against some perspectives, but never because they are not true. Always, it is because they are the product of sick minds.
The principle sickness is spite, vengefulness, which is created by the consciousness of impotence. Find monstrous rage, anywhere in the world, and you will find people who are conscious of their lack of power. But there is more to the story, and this is very important: They let their lack of power define who they are, and the way they see the world. It gnaws at them. They cannot let it go. It is the outcasts in high school who are bitterly resentful at the way they are treated by the social hierarchy, and let that define who they are, so that their very value system is framed as an antithesis for the popular, that they view themselves superior because they recognize the stupidity of the social elite (and they may well be! Truth is not the issue!). Or the popular themselves who need to feel superior to others, a need which manifests itself by putting down the people they hate with a kind of viciousness that shocks, always answerable to deep insecurity. It is people in Palestine, who define themselves though the losing struggle with Israel, alway, always aware that they have lost every battle in the decades long war. It is the people in Israel, who define themselves as a people under siege (doe not that very conception come with it the consciousness of the lack of power?). It is the people in the American South, who remember the civil war, their loss being the defining moment of their culture, and it shapes a hatred for those who won. Etc., etc., etc. From serious politics to trivial social arrangements, the rage comes from the same place: awareness of the lack of power, and that lack defining who they are. The story is the same, because the cause is the same: vengeful spite shaping a perspective, a world view, psychology shaping their interpretation of the world.
The need to feel superior, to demonstrate superiority, is itself the product of the feeling of inferiority, a psychological need. The weak are defined by that need. They may in some ways appear strong. But it is only an appearance. The truly strong can let things go, have no need to feel special, no need to demonstrate superiority. They may do things that hurt others, but are not wracked by guilt about such things, nor do such things as a reaction. If they do something wrong, they learn from it, and move on.