I say then: Walk in the Spirit, and you shall not fulfill the lust of the flesh. For the flesh lusts against the Spirit, and the Spirit against the flesh; and these are contrary to one another, so that you do not do the things that you wish. - Galatians 5:16-17 (NKJV)If you don't have the Spirit of God living in you, you live by the flesh and have no inner war between the flesh and the Spirit. You don't have a choice otherwise, nor would you even be able to comprehend it.
But the natural man does not receive the things of the Spirit of God, for they are foolishness to him; nor can he know them, because they are spiritually discerned. - 1 Corinthians 2:14 (NKJV)
So here's the thing, if you believe humanity to be advanced animals, then you will find material contentment in behaving like an animal. In times of plenty, you'll be an r-strategist, and in times of scarcity, you'll be a K-strategist, but in neither case will you be so by choice, but because that will be what is demanded of you by your environment.
In the flesh, this is all we have to live for. The lusts of the flesh, and their satisfaction, and the ultimately diminishing satisfaction as our dopamine receptors adapt or burn out.
What the Spirit of God gives us is the power to make a choice other than satisfying the lusts of the flesh.
Sure, there are folks without the Spirit of God who deny some of them, and for a time, but the pattern of their behavior is always enslaved to sin.
God's work in our lives gives us ability to be more than just animals, more than just machines satisfying desires.
Jesus Christ is not a killjoy, he's the only way to lasting joy and contentment, both in this life and on into eternity.