Most of what you think and say are things you picked up from others. They are not original thoughts. That is why there is this idea of Rav Nahman of talking with God while being alone and trying to get in real contact with who you really are, and talking with God from your inner essence, not just what you have heard from other people.
Why is authenticity important? After all if one is thinking of it as being important because I picked up the idea from the existentialists, then it in itself is not authentic.
The reason is that there is a discernable limit how much one can pick up until his whole inner essence gets erased. You can see this in undergraduate of high school papers which are often a large mixture of clichés.
You can hear this in people's conversation where almost every word is some "buzz word" they picked up from he media or friends.
I admit even after learning the existentialists I was not very impressed until I heard Jordan Peterson point out some of the good points that some of them brought up.]
