A Moody Character

Because he can't get together with Daphne often because of hanging out with 3 other people in a garish van traveling around solving mysteries.

Or it could be a part of his character as the no-nonsense leader of the group who decides on how to solve mysteries.
 
in case you haven't notice, everyone has stiff and limited expressions, except Scooby.
Limited animation will do that.
Plus the guy barely had a personality in the original series.
 
True. In the first series, Fred would just dictatorially bark orders to everybody else. He was supposed to be the 'serious, stoic' member who was straight-arrow while the others were allowed to goof around, though eventually Daphne and Velma started to become super-straight and narrow as well, when the comedy team of Scoob and Shag started to take off.
 
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