To let something happen presupposes it is already at work and you are not going to make an effort to interfere, alter, nor try to stop it from happening.
It is sort of if you are in the middle of a narrow road and a MAC truck is barreling down the road toward you and for some reason, maybe too many MARVEL movies, you stretch your hand out thinking you can stop it. Then a voice from heaven crescendos, "Move quickly or die."
With an insight, that message stirs fear in your soul and you successfully leap out of the way of the truck as it speeds by just missing you. From this experience, you've learned many things including the will of the voice and its love for the driver to get to where she is going on time. Oh, maybe it loved you too.
So what is this all about? People think they can author their destinies and once they have that down pat author other people's destinies. Yet many of the injunctions of the New Testament is for us to let (get out of the way of) something that is already happening. Take the verse quoted below as an example.
"Let every soul be subject unto the higher powers. For there is no power but of God: the powers that be are ordained of God." (Romans 13:1, KJV emphasis mine)