Selfishness has crept into every chromosome of every person that multiplies with every cell division, the people in this world always have there own purposes of being with someone or doing something, even m not sincere to my parents, friends or family. We all have been so caught up in being unfaithful that when we see even the slightest sign of sincerity (which in fact again has some sort of unfaithfulness in it) we believe that person as being the worlds most sincerest person and no doubt in future we will some where find that the same person has his/her own reason which neither of us realized at that one particular moment.
I know Sincerity exists, because I, for one, am one of those whole SINCERELY does things for others without expecting anything in return. I truly believe we all have sincereity within us, not to mention, our world is much larger than the consumerism selfishness of the First World. If you visit poorer countries you will see the DEPTH of love and sincerity in those communities. Something we ALL have when we are down and out. Materialism has made it hard to SEE this, but there are MANY selfless people who do things out of the kindness of their hearts for others, for no rhyme or reason other than to 'give back' and for the sake of love.
Sincereity is more than just giving, as well. It's feeling. And we are ALL capable of sincerely feeling a certain way and sharing that sincere feeling with others.
I have no doubt in my mind that Sincerity still exists.