I am sincerely happy for the success of others.

There is a half-second before the gladness arrives, and most people have met it. A friend's promotion comes through and something ungenerous flickers first - quick, unwelcome, gone before you would ever admit to it.

Sincerity is not the absence of that half-second. It is what you do in the two minutes after: whether you send the message, whether it is specific, whether you ask about the part they are actually nervous about.

The flicker fades with use, oddly. Congratulate people properly for long enough and it gets shorter every year, until the first response and the sincere one are the same thing.