Life can be a series of actions and reactions. You take action and the world reacts. You receive an action and you react. Those reactions are determined by our guiding principles, our virtues. Every action and reaction is processed through our unique lens.
As a child, we're completely sincere but as we absorb the norms and values of our culture we learn to filter our honesty. We develop a lens to process every action. We learn what society values based on our experiences and adjust our behaviour to be more valuable.
Adulthood then feels like a process of unlearning these rules and re-embracing your childhood honesty.
According to Wikipedia, sincerity is the virtue of one who communicates and acts in accordance with their feelings, beliefs, thoughts, and desires.
Children are as sincere as can be but as an adult, it's a difficult virtue to live up to. It requires self-belief and courage to deal with the consequences of being truly honest. There's no room for hedging bets or hiding failure when you're completely sincere. You have to embrace it all.
According to Aristotle "truthfulness or sincerity is a desirable mean state between the deficiency of irony or self-deprecation and the excess of boastfulness." Irony is a hallmark of our generation and an easy solution for many, myself included. But as I grow older and try to understand what "being an adult" really means the more sincerity seems to be the best and simplest answer. There's no pretence to maintain, no internal debates to be had. There's just you, your truths, and being kind along the way.