Why did Satan disobey God?

Answer 1

Satan was once a guardian cherub named Lucifer. Lucifer was created by God Himself. Lucifer was blameless and perfect in his ways. Hence, he became arrogant and desired to be like God. Due to the evil of the guardian cherub, God cast Lucifer down to the earth. Thus, the guardian cherub, once a friend of God, became satan, the devil, the father of lies, the serpent.

Answer 2

Jewish View
Satan is actually incredibly loyal to God and like other angels, performs his duty without question or issue. Judaism holds that God explicitly created the Satan as he created all of the angels. However, unlike Christianity, Judaism asserts that the Satan never rebelled against God nor does he direct or control evil in the world. The Satan is God's loyal opposition, whose job it is to cross-examine God, His actions, and advocate against humanity. This leads to him generally having the responsibility for arguing that humans should not receive mercy when they arrive in the Afterlife (in contrast to God who wishes to provide mercy). These constant arguments with God could be considered "fighting", but since he is only arguing with God because that was the function he was made to perform, it is not really fighting.

The Satan is one of the few angels in The Bible whose name is never explicitly stated; the Satan is his title, i.e. the Opposer, just as the Healer is Raphael's title.