Actually, it's a comfort thing. Studies have shown that people are more comfortable talking to another person when they both have the same posture. It's a tip for dating -- when meeting or conversing with somebody, try to match their position, or somewhat match it. Don't cross and uncross your arms every time they do, for example, but do position yourself similarly. This subconsciously makes people more comfortable.

And yawning ... yeah, that sucks.