Simply to much light, peace lilys like warm, shaded places, they also like nicotine as bizarre as that sounds, so if you know a smoker get them to puff on your plant. If your not sure on watering, simply fill a bowl, put plant in the water (water should rise half way up the pot) leave for 15 mins, and allow to drain, repeat whenever soil drys. Doing it this way the plant will take all the water it needs.