The law of cause and effect is the prime law that runs the universe. It is the number one law. Every spiritual and scientific teacher has sought to teach it. They may have said you reap what you sow, or you get what you give, or what goes around comes around, or karma, or consequences, or every action has an equal and opposite reaction, or many other similar statements. Quantum physics is now teaching us how this works, exactly, on a sub-atomic level. Things get better when you get better. They get worse when you get worse. The world is all within you. You are at cause over everything that happens in your life, whether you were conscious of it or not. To be happy, cause another to be happy. If you wish to have peace, teach peace to learn it and have it. Cause and Effect is so for many reasons, such as (i) it is the way ideas grow, and everything is literally an idea, Thought extended and manifested, (ii) your world mirrors your thoughts and beliefs, and (iii) whatever you hold as real outside of you is planted as real in your mind, and what is in your mind will affect you someday in someway unless you change your belief in it.