Every pastor and probably every Christian has known a fool who is on a sinful, idiotic, destructive course. (Which is to say: being a fool.) From where you're standing, it doesn't even make sinful sense. Every sane person you know looks at it with slack jaw. The Bible is crystal-clear — which, if the person professes faith in Christ, is supposed to mean something. Any flow-chart starting where that person is ends in disaster, barring a sharp 180 sometime soon.
Yet on he or she goes, grimly determined and undeterred, stubbornly resisting every Biblical plea and warning, long after he can rub even two rational defenses together. The flight from God persists.
Why?, you wonder until your brain itches and throbs, and the tears flow.
The answer is as simple as it is unsatisfying: he does it because he's convinced it will bring him delight.
And that explains every sin, from Satan's rebellion to Adam's burbling idiocy to Judas' treachery to Right Honorable Reverend A. Postate's latest "discovery" to your and my "li'l peccadillo." Every sin. The sinner does what he does because he is convinced that it will bring him happiness, delight, joy. It seems right. He doesn't care where it ends.
That is why he persists, long after his life falls to shambles and he sees his own character collapse like the two towers in New York. That is why all the pleas and warnings and tears of his Biblically-faithful friends fall on deaf ears. He is like the ruined gambler, absolutely convinced that the next dollar will bring untold riches... or the next dollar... or the next dollar....
As with most (all?) such things, it reveals defective faith. If the person claims to be a Christian, he either does not heartily believe what he says he believes, or he doesn't really believe it at all. He calls Jesus Lord, but does not do as He says (Luke 6:46). He calls God his fountain of joy and life, but seeks joy and life away from Him and in defiance of Him.
What to do?
For ourselves: pray, watch, stay humble and rebukable.
For others: the same. Point to Christ. Point to Scripture. They may flee to a thousand evasions (check out this post for reminders and specifics). Don't weary, don't lose heart — you wouldn't want someone to lose heart with you if you strayed, would you?
But over all and above all and through all: pray. It takes the Spirit of God to transform our desires. By definition, we can't do it ourselves. Our wills are free to choose according to our nature, but our wills are not uncaused causes. We need the Spirit of God to transform us, so that we begin to desire what we do not now desire, and no longer desire what we once did.