Another question might be: Why shouldn't you? Who wants to be around a Grumpy Gus or Debbie Downer. People feed off your energy. If you are always a negative person, no one will want to be around you - they don't want to be taken down by your negativity. It's too depressing.
When you smile and portray a positive attitude, people will smile back and be more apt to engage you in conversation if you appear approachable.