Pretentiousness is the opposite of sincerity in my opinion.
It never ceases to amaze me how someone can be so calculated in their responses, yet so uncaring at the same time.
Our actions speak louder than words. Someone who is sincere will back it up with what they do, not just talk about something with empty words.
I have a huge extended family. 100+ people. When I was hospitalized with paralysis, and then couldn't walk for several months afterward, a total of about 5 people actually cared. One brought a couple of casseroles and meals. One came to visit once. I got one get well card. My grandfather, sick with cancer, was the only one that consistently helped us. He was a tremendous help with the children as well as helping my husband with things I could no longer do.
It was shameful and sad to me because I have plenty of relatives, right next door, that didn't even glance in my direction as they drove by. Never once offering to help. Yet, posting on FB questions about how I'm doing. And I don't even use FB. That's pretentious and insincere. (And then, becoming upset because after I could walk again, I didn't send a gift to a shower.)