As per sacred texts of India: Shiva is generally known as destroyer of the universe, one of his symbols is moon. He is also one who has received celestial river Ganga upon his head, thus saved the earth from destruction and gave opportunity to us mortals to bathe in this celestial river - thus purifying our selves. He is also known as "Ashutosh" one who can easily be made angry and as well easily pleased (who then easily grands almost any boons to his worshipers). Shiva-lingam is a symbol of creative potency and often worshiped by bathing (here in milk), thus making Shiva pleased, pacified.