This might be a basic answer not befitting Stack Exchange (but it's a basic question):
You want to buy things for a price as low as possible. You want to sell things for a price as high as possible (you probably only sell labour, unless you run a business). Those two pressures find a balance at some particular price, and that is the price.
Anyone can open a store that charges \$30 for a loaf of bread, but nobody would buy one. Anyone can walk into a store and try to give them \$0.10 for a loaf of bread, but nobody will sell one. Somewhere in the middle, there are prices where you want to buy bread, where the store also wants to sell bread. They can charge any price in that range.
Why wouldn't they charge the highest price they can get you to pay? Two reasons:
Different people are willing to pay different prices - it might be if they make the bread 50 cents cheaper, they sell a bunch more bread to make up for it.
Stores compete with each other - it might be that if they make the bread 50 cents more expensive, you'd still pay it if you had to, but you don't have to because it's 50 cents cheaper at the store next door.
If you think about it too hard, it's amazing that things actually work this way. I remember hearing once that after (or maybe just before) the USSR fell, some USSR government employees who were in charge of setting prices visited Britain, and they were confused by how the economy could possibly work in such a disorganized fashion. Yet here we are and it works.
In the context of taxes, it is somewhat true that taxes tend "spread out". If you tax stores more, for example, stores will charge higher prices—they have to, or they wouldn't make a profit any more—so you'll end up paying more for bread. Even though you wanted to tax owners of stores, you ended up taxing everyone. It's not entirely true, because the tax money can come partially from higher prices and partially from lower profits at the same time.