For non EU citizens, visa runs can be something of a pain. Your short stay visa only entitles you to 90 days in the Schengen area in a six month period. At the end of this 90 days, you don’t just need to leave Spain for the rest of the six months, but the whole of the Schengen area. The nearest non-Schengen spots are the UK and Ireland, north Africa, Switzerland (though it is expected to join), and the countries that used to form Yugoslavia.