My most cynical take on John's "charm" is that it's a self-fulfilling prophecy. (Partly because to me, he comes across as smarmy and off-putting in a lot of these moments when he's meant to come off as charming.) He's a good-looking white guy from a moneyed background, and he's used to positive attention, so he feels certain of his welcome and projects confidence, which people tend to find charming.

But by the same token, going by his behavior, John seems like maybe he expects people to be receptive to him on a superficial level, but hostile to any deeper thoughts or emotional expression on his part, which might explain why he finds it so agonizing to share those things.