Why

I'm not sure. Mine do it too, though.

 

Yes! It's a sign of contentness, lots of different birds do it. If you ever had a parakeet or cockatiel you have probably heard that sound they make (It sounds like a finger nail on a rough surface) with their beak when you give them a head rub, or right before they go to sleep.

 

I've always thought that smacking lips is a remnant behavior from chickens that see you as "mama" left over from when mama birds fed their young before chickens were chickens.