Because the carnal mind is enmity against God; for it is not subject to the law of God, nor indeed can be. So then, those who are in the flesh cannot please God. Romans 8:7-8
In the ancient world, you would never see an ox and a donkey yoked together, pulling a plow. That stipulation is found in the Old Testament along with others about mixing unlike things together (Deuteronomy 22:9-11). These stipulations reflected a principle for the Israelites: Don’t be yoked together with unbelievers in Canaan. That principle made its way into the New Testament in 2 Corinthians 6:14-18.
But it also reflects an even more impossible mixture: the Holy Spirit and the carnal nature of man (the flesh). When we are saved, we are delivered from the power of the carnal, sinful flesh in our life; set free to live life powered by the Spirit of God. The flesh and the Spirit are like oil and water—they simply cannot be mixed. For a Christian to manifest the nature of the flesh is a signal that something is wrong.
Life in the flesh can never be pleasing to God. It is what Christ died to deliver us from! To please God, live a life yielded to the power and guidance of the Spirit.
The flesh is radically and wholly evil. A. W. Pink