Then Jesus said to his disciples, “Whoever wants to be my disciple must deny themselves and take up their cross and follow me." (Matthew 16:24)
As Christians, I am afraid that we often over-spiritualize the term "flesh". While it is true that flesh means more Biblically than just our skin, it is important to realize that our physical body still plays a huge role. Flesh may not be the whole team, but in certain games, it is the star player!!
I would like to submit that no one understands the meaning of "deny themselves" better than a person who has overcome a biological tendency for something*. Drugs, alcohol, food, lust, anger etc. The predisposition to perform particular sinful acts is inherent to every person. And though we may not realize it, many of them are prompted biologically. The most obvious example is of a person who has become addicted to a substance and developed a biological dependency. That is a situation that most people know about -- dependency, withdrawals, relapse etc. Less obvious examples would be things like: the chemical reactions in the brain that make a person anxious, stressed, on edge etc. That person's body is "setting them up" for whatever sin that they most readily fall into (i.e. anger, depression etc). The same can be said of a physical impulse for sexual desire. Again, the body acts as a sort of "setup", making the choice to embrace lust via pornography or adultery seem practically inevitable. Food and alcohol are doubly dangerous in this regard because the body prompts a person to eat/drink and then whatever is consumed produces chemical reactions that can prompt in other ways. Obviously, I am not a medical doctor and am speaking from very limited and general knowledge about this sort of thing. Nevertheless, I think these things are important to understand, because things that are prompted biologically can easily be excused. "It just comes naturally," as the saying goes. In truth, all sin comes naturally.
My point in saying all of that is to draw attention to something. Here is an example. It is no accomplishment for me to say that I have never used heroine. I have never wanted to use heroine. I have never had to "deny myself" heroine at any point. I have never even seen it!! Unfortunately, there is an epidemic of morally superior Christians who live their lives with a sense of false pride because of all the things that they "don't do", (i.e. don't smoke, don't drink, don't gamble, don't cuss, don't beat my wife etc). But if you talk to Believer who has trusted in God to overcome a biological dependency on something, they are usually not particularly prideful about it. It was a painful and humbling process. They know what it means to truly "deny themselves".
Let me encourage everyone. Do not be prideful of the things that you "don't do". And do not sweep under the rug the sins that you are giving into regularly just because they seem "natural" or because they aren't as publicly offensive as drug abuse.
Galatians 5:16-17 "So I say, walk by the Spirit, and you will not gratify the desires of the flesh. For the flesh desires what is contrary to the Spirit..."
*Don't misunderstand me. Everything comes back to a spiritual struggle. There are innumerable, deadly serious, struggles that are not as easily linked to biological tendencies. Either way, we must learn to live by the Spirit to overcome the flesh.