Why do Christians still struggle with sin?
Christians still struggle with sin because the flesh, called old Adam, lusts or wars against the indwelling Spirit, and the Spirit warreth against old Adam. This internal conflict continues until they walk in the Spirit, which enables them not to give in to the old desires. The one you feed the best wins.
Les explains that the struggle is between the flesh (old Adam) and the indwelling Spirit. The flesh lusts or wars against the Spirit, and the Spirit wars against old Adam, so they are in conflict. He illustrates this with a saved alcoholic who had to pass his old bar; he felt like two little dogs inside fighting—a black dog urging him back to his old lifestyle and a white dog urging him to go get his mail. The one that wins is the one he feeds the best. Walking in the Spirit, that position of being in Christ, is how believers avoid giving in to the old desires of old Adam. When they don't, disappointment follows, and God can be disappointed too.