Because this is very important. It's very important because if your intention is to 'share', then the person listening to you is probably a) somebody with whom you're very comfortable, and b) not somebody you're looking to impress.
On the other hand, if your (usually unconscious) intention is to 'talk about yourself to get attention etc.', then chances are the poor person who's listening to you is either a) somebody you couldn't care less about (the Samaritans hotline would provide just as commendable a service), or b) somebody you really want to impress and therefore cannot be totally un-self-conscious with.
Which kind of presents a new problem : that those we wish to impress will be the ones we find difficult to 'share' ourselves with (at least until, I don't know, ten years later when the honeymoon period dies its natural death and you're both sufficiently attuned to each other's digestive noises), which means that you'd bloody well better be with somebody you don't fancy too much and, goodness, if there's somebody you really fancy, you'd better run like hell.
P.S. - I'm quite sure Freud came up with the same conclusion; something about man not being able to desire what he loved and not being able to love what he desired, which makes me think 'really now?' and at the same time 'uuuggghhh', because the Freudialization of today's world is so near complete it's unbearable.
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