The heart is deceitful above all things, and desperately sick; who can understand it? (Jeremiah 17:9; ESV)
Humans are complex creatures. That shouldn't be surprising, since we are made in the image of a complex God. Scholars debate what being made in the image of God is all about. It's difficult to determine, since as far as I can tell, nowhere in Scripture is it clearly defined. One way we might go about understanding this is by looking at how the Bible describes both the characteristics of God and of humans. I think it is reasonable to assume that those human characteristics that reflect God's own are due to being made in his image. Where else would they come from?
A related topic has to do with how sin and its consequences affect God's image. Some think that evil has completely eradicated it, but I don't think that is correct. The prohibition against murder given to Noah is based on our being made in God's image (see Bereshit / Genesis 9:5-6). I believe it is most correct to say that the image of God in people is still present, but is essentially marred by evil. This helps to explain the reality of goodness, love, and compassion in human experience and our continuing struggle with selfishness, pride, greed, and evil fantasies.
One of the common characteristics shared by both God and people is desire. Both God and people have the capacity to want. The difference between us and God, however, is that while his wants are completely pure and good, ours are mixed at best. People can and do have true godly desires, but sin constantly pulls us in a selfish, evil direction.
If we take the verse I quoted from this week's Haftarah out of context, letting it stand on its own, we might conclude that our hearts are such that it is impossible to have pure desires of any kind. But I don't think that is what God is saying through the prophet Jeremiah here, especially if we look at the verse's context. The point being made here isn't so much that we can never desire anything good, but that the conditions of our hearts are such that we cannot accurately discern our motives. This means we, just like God, are filled with desires, but the desires themselves provide no help whatsoever in determining which ones are good and which ones are not.
It is in this same book of the Bible that God gives his people a most wonderful promise that addresses this. The resolution of the sin problem that prevented Israel from being the people God called them to be would be accomplished through the internalization of Torah (see Jeremiah 31:31-34). Properly understood, Torah is the teaching and direction of God. God promised that he would write his Torah on our hearts. This is similar to Ezekiel's prophesy about God swapping our heart of stone for a heart of flesh, resulting in a new ability to obey him (see Ezekiel 36:22-31).
But does this experience mean that now our hearts can be trusted? Does the transforming work of God through the Messiah Yeshua so completely heal our diseased hearts that the sheer presence of a desire indicates a godly want? If so, then we should be encouraged to follow our hearts wherever they may lead us. After all didn't David write "Delight yourself in the Lord, and he will give you the desires of your heart" (Tehillim / Psalm 37:4; ESV)? Certainly he did, and I do believe that if we delight in the Lord, our desires can reflect God's own heart. But remember what the issue is in our beginning verse: it is the heart's deceitfulness. One of the ways this deceitfulness is expressed is by telling us that every desire we have should be acted upon. While through the Messiah the Torah is internalized, this doesn't make every impulse we have automatically good and godly. Through the Scriptures we learn God's ways. By his forgiveness and the power of the Ruach HaKodesh (English: the Holy Spirit) we are empowered to live godly lives. But we need to pay attention to his direction. Otherwise we are simply driven by desire, not led by God's Spirit.