Sometimes there just isn't enough room!  No matter how you use the rooms or how often you rearrange the furniture, you just can't do everything you want to do with your current floorplan.  That's when the best solution could be a room addition or second story.  It's nearly always less expensive than buying a bigger house and it certainly eliminates the hassle of moving to a new location.  There are also tax advantages of adding on rather than moving out.