Why?! Because we are human and fall into our old man nature (our flesh)! Galations 5:17 says that the desires of the flesh and the spirit are opposed to one another. Verses 19 & 20 speak of the works of the flesh. I challenge you to find one positive thing about them...you can't. The fruits of the spirit are love, joy, peace, patience, kindness, goodness, faithfulness, gentleness, self control, and things like these. Now think of the opposites of those words (hate, sadness, turmoil, impatience, cruelty, evil, unfaithfulness, harshness...) and you have the flesh. I run into these words often in my life!
The mind set on the flesh is hostile to God, it does not submit to his law, indeed it is unable to. And it can not in any way shape or form please God. But the mind set on the spirit is life and peace! What fun is it to be impatient, to think evil of someone, or to be harsh with others anyway? To live in the flesh produces death but to live in the spirit produces life...
Here are just a few examples from Proverbs:
Proverbs 14:30 "A tranquil heart gives life to the flesh, but envy makes the bones rot."
Proverbs 17:22 "A joyful heart is good medicine, but a crushed spirit dries up the bones."
Proverbs 11:3 "The integrity of the upright guides them, but the crookedness of the treacherous destroys them."
Proverbs 11: 8 "The righteous are delivered from trouble, and the wicked walks into it."
Proverbs 11:17 "A man who is kind benefits himself, but a cruel man hurts himself."
Proverbs 11:19 "Whoever is steadfast in righteousness will live, but he who pursues evil will die."
Proverbs 11:27 "Whoever diligently seeks good seeks favor, but evil comes to him who searches for it."
Proverbs 12:18 "There is one whose rash words are like sword thrusts, but the tongue of the wise brings healing."
Proverbs 12:20 "Deceit is in the heart of those who devise evil, but those who plan peace have joy."
Proverbs 12:21 "No ill befalls the righteous, but the wicked are filled with trouble."
Proverbs 12:28 "In the path of righteousness is life, and in its pathway there is no death."
Proverbs 12:3 "No one is established by wickedness, but the root of the righteous will never be moved."
Proverbs 4:23 "Keep your heart with all vigilance, for from it flow the springs of life."
This next part is information I gathered out of "Sparkling Gems From the Greek," by Rick Renner.
"But I say, walk by the spirit and you will not gratify the desires of the flesh."
The word walk in this verse is the greek word peripateo. Peripateo is a compound of the word peri and the word pateo. Peri means around, something encircling, or concerning. Pateo means walk, movement of feet, step, stride, tread, or trample. Put the words together and they mean to frequently, continually, or consistently, walk around the same area. So instead of walk by the spirit the verse could say live in the spirit. Do it so consistently and continually that it becomes a habit; your way of life.
If we are of Christ Jesus we have crucified the flesh along with its passions and desires (Galatians 5:24) and we've been set free (Galatians 5:1)! Let's choose life! And make a concentrated effort to walk/live by the spirit.