The way of Cain is the way of sadness.

In Genesis we have a situation where "by faith Abel offered to God a better sacrifice than Cain" (Hebrews 11:4). As a result Cain's "countenance fell"(Gen. 4:5 NASB); or as the NIV puts it, "his face was downcast".  Cain was depressed and without joy, because of his failure to have faith in God, which is evident from the fact that God was not pleased with his sacrifice.  This is clearly seen in verse 7: God says to Cain, "If you do well, will not your countenance be lifted up?".  This is a rhetorical question given to Cain, but this can be applied to all humans.  "If you do well"--  Abel did well because he offered the best of his flock to God.  He not only understood that there needs to be a sacrifice for our sin problem, but by giving the best of the flock, Abel is showing that his contentment, satisfaction, and even delight is in the Lord.  "If you do well"-- if you put your hearts content, satisfaction, and delight in the Lord then "will not your countenance be lifted up?"
 The heart needs to filled with something, and that is why God warns Cain: "if you do not do well, sin is crouching at the door; and its desire is for you"(v7).  If we do not fill ourselves with the God, then sin will certainly overtake our hearts.
Our hearts will never no true joy, peace, satisfaction, and fullfillment until we look to God to be our treasure.  Oh Christian, if you are in sin right now then you know what I am talking about; how when we stop seeking Jesus, then we lose the joy that we had in God, and sin is there desiring to consume our hearts.
So may we not follow the way of Cain, who did not put his faith in God, and as a result was given over to depression and sin.  Let us live as we were created to live.  Let us worship our glorious creator, let us "draw near to God and He will draw near to [us]" (James 4:8), and in doing so our countenance will be lifted up.