Once the French actor Jean Gabin made a song "Maintenant je sais", "Now I know". He tells a story that when he was young, he always thought he knew everything, and as he grew up, he started to doubt what he really knew, what he really understood of life. He concludes saying as a 60 year old, his life is mostly behind him and "there is only one thing I know for sure: I never know".
Maybe as you are young and are trying to find your way in life, you need to reassure yourself not to be swept off your feet by everything happening in life. And as years go by, you learn about your own strengths and weaknesses, so you no longer need to hold on to the straws of false assurances... So you can give yourself the liberty or privilege of doubting.
Another way to look at it, is: when you are young, 8 or 10, you look up to adults as if "they know what they are doing". A job, kids, house, financials, life in general. As you grow into an adult yourself, you start to see the doubts and struggles that also your parents have experienced: they did not know neither, but tried their best.
I will turn 50 this year, believe it or not. When I was young I always said I would die falling of a tree, a cliff, freeze to death on some mountain top, crash in a remote area in Africa before I turned 50... I never believed I could turn 50, me, who was always the youngest and the wildest in the bunch...
But now I do turn 50, I also learned that struggles and doubts continue if you live life intensively and to the fullest. I know these internal battles will never stop. I learned that bit, and came to terms that "I will never Know"... I will continue to doubt whether the choices in life I am making, are the right ones for me, for those around me. Whether the choices I make at work are the right ones, whether I do things right. And somewhere that is the beauty of life. And maybe it is the strength of a person: the strength to dare to doubt. The strength of understanding you will never know.