"Do not drink intoxicating wine, you and your sons with you, when you come to the tent of meeting, that you do not die, this is an eternal decree for your generations." (10:9)
Aaron was commanded not to perform the service while intoxicated. What was the reason for this prohibition? Why is a Kohen forbidden to perform the service after drinking alcohol?
The Torah and its commandments are here to provide us with pleasure and fulfillment in life. As the verse states, "The orders of God are upright, gladdening the heart." (Psalms 19) God wants his servants to find their joy in the Torah and the performance of its commandments, not through external stimuli. A Kohen who performs the service while intoxicated fails to find gladness solely through his service, and is therefore deficient.
God created us in order to bestow us with pleasure. Just like our parents want us to have everything that is good, the Almighty wants the same for us: to enjoy the world, and get as much pleasure as we possibly can! However, he wants us to have fulfilling, long-lasting pleasures and not fleeting, meaningless pleasures. This deep pleasure is attained by attaching ourselves to Him. That awesome feeling one has when helping another person or after a good prayer is the feeling of connecting with his soul - connecting with God. Alcohol, drugs and all other physical pleasures are cheap imitations of this enjoyment. Why not go for the real thing?