Carl Sagan's 'Cosmos' explains stuff like this better than any textbook. If you can find the old TV series or the book, watch or read it. He had a gift for making the universe make sense without dumbing it down. Highly recommend.

The honest answer is that scientists don't fully understand this yet. We have good models that make accurate predictions, but the 'why' underneath is still an active area of research. Anyone who tells you they have the complete answer is oversimplifying.

This is something I struggled with until a professor explained it using an analogy that finally clicked. The real world is messy and doesn't always fit neatly into the simplified models we learn in textbooks.

The math looks scary but it's mostly just a precise way of saying something you can understand in plain English. Once you get the concept, the equations are just bookkeeping. Focus on the idea first, the symbols second.