God is a jealous God

   Exodus 20:4-6  Thou shalt not make unto thee any graven image, 
   or any likeness of any thing that is in heaven above, or that 
   is in the earth beneath, or that is in the water under the 
   earth: {5} Thou shalt not bow down thyself to them, nor serve 
   them: for I the LORD thy God am a jealous God, visiting the 
   iniquity of the fathers upon the children unto the third and 
   fourth generation of them that hate me; {6} And showing mercy 
   unto thousands of them that love me, and keep my commandments. 

   Exodus 34:14  For thou shalt worship no other god: for the 
   LORD, whose name is Jealous, is a jealous God: 

   Deuteronomy 6:15  (For the LORD thy God is a jealous God among 
   you) lest the anger of the LORD thy God be kindled against 
   thee, and destroy thee from off the face of the earth. 

   God expects faithfulness.  Faithfulness in heart and action.  
   He wants love and obedience.  He does not take well to the 
   cheating heart.  Just as we do not take well to a cheating 
   heart in our spouse he does not take well to a cheating heart 
   either.  Just as we are not able to accept infidelity in our 
   spouse he does not accept infidelity either.  

   God is a jealous God.  He does not take it well when we start 
   listening to Satan (his enemy), inclining our ear and heart 
   toward Satan.  Just as a person becomes upset and jealous when 
   his spouse shows signs of unfaithfulness and starts flirting 
   with other people even so God becomes upset and jealous when we 
   show signs of unfaithfulness and start flirting with Satan.  
   God represents Goodness, Virtue, Morality and Right.  Whenever 
   we start considering Wrong, start listening to Wrong, we are 
   really listening to the voice of his enemy, Satan.  When we 
   turn to God, when we commit ourselves to him, we become married 
   to him in a sense (similar to the way that a man becomes 
   married to a wife --- we enter into a covenant or "marriage 
   contract" with him).  Engaging in sin after this is an act of 
   unfaithfulness, infidelity and untrustworthiness on our part.  
   God designed us to do Good.  He made us with the intention that 
   we would follow his way of Goodness, Love, Right, Honesty, 
   Morality, etc..  That is the way we were designed to go.  To go 
   the way of Evil is against our basic nature and design --- and 
   it is disloyalty to the God who made us and loves us.  Satan is 
   a liar who is continually trying to deceive us and divert us 
   from God's way.  He is a tempter.  He would try to get us to 
   believe that his way is better.  He would try to get us to 
   question God's way and to doubt it.  He wants to pull us away 
   from God, pull us into his way.