A Justified (but not True) Belief can fail to be true for at least 2 reasons: 1) because we lack the evidence or awareness of the evidence to justify the actually true belief, or 2) because we have chosen poor justifications, such as logical fallacies.
Logical fallacies are reasoning that have provably failed to determine the truth of a hypothesis or belief, and are therefore not useful for determining the truth. Example: "Well Socrates thought the sun revolved around the earth, and he was a smart guy." Fallacy: appeal to authority.
A True Belief, while true, is indistinguishable from a False Belief if it is not justified. Just as many once thought the sun rotated around the earth and believed fire is hot, we can have true and false beliefs. We need justification to separate the wheat from chaff.
but I don`t know why assumptions are something bad or contrary to true believe ? When I say the water is blue, the grass is green, the tree is brown etc. and a lot more. Why is it bad ? I think the mistake are metaphysical premises but not assumptions.
Grass is green is a statement justified by experience. It also can be justified by measuring the wavelengths of light it reflects. Those statements are not an example of True Belief lacking justification. You can't actually give me an example, because we can't know its true without justification.
is only word-play: when I say, the water is blue, then I make a assumption (Behauptung). So, we make very many of them. And that`s not wrong. I know, because we can justify them. But the richness of assumptions don`t make the world unbelievable (they aren`t metaphysical assumptions).
Mystery is usually nice due to the promise of the thrill of discovery, or do to fear of knowing the truth. The first shows that mystery is just an intermediary step toward understanding, and the second is an unfortunate and probably unhealthy condition.
Those who have an accurate understanding of the world are more capable at navigating dangers and effecting change. Meteorologists who don't understand what temperatures cause rain to become snow are going to be less effective at their job than those who do.
Edit: You ask "why should we want to understand the world ´better`?" So you suggest (because of the `better´) a positive answer. If you erase the word `better´ you have this: "why should we want to understand the world." This is more neutral. Because know I could say this...
I don`t know how the knowledge of black holes, lead to more empathy. Nor the knowledge of psychological priciples, or something like that. Empathy is based on sympathy, sympathy is based on subjective evaluation, but not on facts. Even this fact doesn`t lead to more empathy.
but JB could also leads to the other way, to less empathy. (The notion of Determinism, the notion of philosphical zombies, the denial of free will). That doesn`t make your statement untrue. But it is warn sign. We shouldn`t euphemize JB.