How come when i sync music with wmp it automatically deletes stuff on my mp3 player that used to be on my comp

i go to put stuff on my zen with wmp by selecting all the music im about to put on and selecting add to windows media player list, then i go to the Sync pulldown and select add now playing to my zen, it syncs it like normal, the new songs are there but when i try to play old songs that used to be on my computer theyre not there, i think wmp deleted them without telling me. I tried to delete all of the folders that it monitors but it still deletes them, HELP ME!!!!