Our politicians have got it into their heads that they should always smile at the public, no matter what they're saying. Theresa May was at it last night- smirking through a piece to camera about how she's going to cut the police budget. It was wholly inappropriate. Serious talk demands a serious face. Smile when there's nothing to smile about and you don't look friendly and reassuring (which is presumably what they're aiming for); you look like a creepy underling trying to suck up to the boss.
It's the Uriah Heep effect.
David Cameron doesn't do it- he's one of the few with the PR skills not appear to be doing PR- and that's almost certainly why he's prime minister.