If the paint is not flaking or peeling then you don't need to remove the old paint before repainting over it.
Usually you can just apply a primer over the old paint and then repaint over the old paint without a problem and still get a pretty good finish.
If the paint is on the outside and it's on wood then I would recommend sanding down the boards to get as smooth of a surface as possible and then apply primer and then apply the new paint.
On interior walls I just paint first with some primer and then the next day I paint over the walls without removing or sanding down the walls and still get a good finish on the interior walls.