Sincerity stems from authenticity, authenticity from the self. The idea of being authentic to yourself implies, to me at least, that you have a set or sets of guidelines that your existence follows when it interacts with the world, and with your inner world respectively. The guidelines are not necessarily the same for the self and the interaction with the other. The differing sets of values can arise because of a variety of cultural or interpersonal reasons-say a strict childhood, oppressive regime, or anything in between.

So then there lies a problem of defining sincerity, you run the risk of becoming sort of a Cassandra, if you will-when one is sincere according to the guidelines their exterior self presents, the only one who does not believe themselves is themselves, and when one acts according to the values they may truly believe in, the people around them react with disbelief and anger. A lighter version of this phenomena may be experienced by someone who is "breaking the mold" of what they have previously done in favor of something new.

Sincerity we can then say is the act of ones inner values and external schema being in alignment with each other. It is important for the self, as unless you are amoral you will begin to build resentment of yourself. This resentment degrades you into a weaker and weaker self, and thus you will eventually lose yourself.

Fascinating post Jakus.

Thanks.

My take:
Insecurity breeds insincerity, the thought of being sincere means being open and that could lead to being taken advantage of. Keep the defences up by keeping people guessing. But for most it fades away with time, having the security of loving folk around you makes you realise it's safe to be open with everyone.

So, then. How does one become more sincere? Is that possible?

Time I suppose, time to gradually emerge into an optimistic way of life?

Can attitudes like sincerity be shaped and moulded or are they static?

I think they can develop over a lifetime of experience and reflection. Or remain static if we are too lazy to accept change, a stubborn refusal to acknowledge we were wrong and move on.

Sincerity is simply forthrightness or honesty. In that respect, few of us are always sincere.

Sincerity is simply forthrightness or honesty. In that respect, few of us are always sincere. — gloaming

Yes; but, it's more than that only. It's also an adherence to the commands of good intention. Whatever that may be.

Honest behaviours are always motivational...intentional. So, yes, there is adherence, but I'm not sure people adhere because of the 'rule', or because of the proscription for not having that orientation in the Kantian sense (if that's what you mean by saying "...the commands of good intention.". They may just adhere because it's their orientation prima facie. They may do it because they're in love with the person. They may do it out of fear of being caught out in a lie. They may be harkening to that still small voice.

but I'm not sure people adhere because of the 'rule', or because of the proscription for not having that orientation in the Kantian sense (if that's what you mean by saying "...the commands of good intention.". — gloaming

Yes, this is the crux of the issue. Do we do it for some personal satisfaction or for an ulterior motive? Being sincere should only mean being drawn towards doing what is good for its own sake, and for no ulterior motive.

One can always lie about one's motives; but, that can only last for so long before being discovered by other minds.

People don't always know their own motives. I agree with gloaming (assuming I understand the comment correctly) that it doesn't happen only under a single set of conditions.
Speaking in bad faith certainly leads to poor discussions. But not all "good faith" discussions are worthwhile. What I ask myself a lot is if I am really having the problem being discussed. In that register, sincerity is a desire to go somewhere, closer or further, to or from something that a thinker thinks is important for themselves and who they love.

I believe all acts of charity and good deeds are required to have an element of sincerity to them. I might be trivializing the issue into conflating sincerity with compassion or empathy; but, I don't see how you can be ethical without sincerity.

It can be argued that sincerity is a basic desire from which emanates the good of man.

Thoughts?

Some people sincerely believe bad things. Not just in the sense that they are wrong (a discussion in itself) but that those beliefs make them whole, in their mind.
People make choices. The possibility that gives you happiness is handing out other outcomes at the same time.
The importance of the theme of self knowledge is that it is the only thing pushing us toward taking responsibility for what happens.
And thus all the arguments about what is true for oneself is immediately entangled with other people's truth.

Some people sincerely believe bad things. — Valentinus

That's true.

So, sincerity alone isn't enough, as usual?

Or maybe it is a viable idea but needs further qualification.

Or maybe it is a viable idea but needs further qualification — Valentinus

Please expand.

I see the matter as what Kierkegaard refers to as "earnestness" He wrote:

"Therefore nothing can be said about earnestness in general. It is not pure subjectivity or any similar stupidity. Earnestness is present only in the very finest concretions (the empirical self) and as a qualification of freedom. To speak of freedom in any other sense is a misunderstanding. There is no measuring rod more accurate for the determination of the essential worth of an individuality than that of learning what in a pregnant sense made him earnest in life, for with a certain kind of earnestness one can deal with various things, except that from which an individual dates his life. Earnestness about the national debt, about one's own debt, or about astronomy, etc. A healthy spirit manifests itself precisely in being able to deal with everything just as sentimentally as jocularly, and just as well. But in relation to earnestness, it tolerates no sentimentality and no joking. If it does that, it will happen to such a person as with Albertus Magnus, who boasted of his speculation and suddenly became stupid."

From Paper V B 65. as a note in Concept of Anxiety translated by Reidar Thomte and Albert Anderson.