Anime: Fullmetal alchemist (Fullmetal Alchemist). Page 7

- The Dog... Wonderful!
Really?
- Of course! They are loyal, unquestioningly obey any order of the owner, do not complain if you are cruel to them, do not require salaries. In short, a real servant of man! How lovely, I love dogs!

Nothing goes for nothing. If you want something, you have to give something in return of equal value. In alchemy this is called the principle of equivalent exchange.
Our pain - it is also the equivalent charges. But the world is not perfect. Therefore, it is beautiful and extraordinary. Who wants to have to pay their labor, blood, sweat. The law of conservation manages the world only partly. The rest of the man himself.