Why are some people just great at building Rapport?

[Richard]

True isn't it.

Some geezer's just seem to have it. What is it about them, what are they doing. It just seems to come natural to them.

Yet there are times when even the most unlikely candidate just seems to switch into gear and appear perfectly at home chatting and effectively relating with little or no effort. Then someone else comes in and they just seem to lose it. Strange, or not really? What do you reckon?