The short answer is “If you say it, then you prove it”. That seems fair, right? But do all cases have to be proved “beyond a reasonable doubt”? The answer is no and the reason why makes sense.
The United States Declaration of Independence declares “Life, Liberty and the Pursuit of Happiness” as inalienable rights. The 5th Amendment offers protections to our “life, liberty, or property” noting that these rights cannot be taken from us without due process of law.
If your life, liberty or pursuit of happiness is at risk then you are most likely a criminal defendant and it is the government that is trying to take these rights from you. As such, the burden on the government should be as high as possible to protect your rights. As everyone has heard in nearly every movie or TV show, that burden is generally stated as “beyond a reasonable doubt”. That makes sense too.