The letter S in a life lesson is normally picked for success. But yours truly has a humble opinion that sincerity is one of the key steps that makes one's life successful in one's own eyes if not in the world's eyes.
In fact, Oscar Wilde cautioned us,
" A little sincerity is a dangerous thing,
and a great deal of it is absolutely fatal"
However, Charles Dickens held out for the other view in these words
" There is no substitute for thoroughgoing, ardent
and sincere earnestness"
However, Mehmet Murat made it even more simpler by saying
" Paradise can take the form of anything!
It can take the form of a flower
or
it can be a word
or
it can just be a sincere smile"
Ralph Waldo Emerson pointed out
" Sincerity is the luxury allowed, like diadems and authority,
only to the highest rank.
Every man alone is sincere.
At the entrance of a second man, hypocrisy begins"
But the resolute belief is in the words of Jean Giraudoux