Here is the quote from Boice and Ryken’s The Doctrines of Grace I gave at the conclusion:
In the natural world there are animals that eat nothing but meat. They are called carnivores, from caro, carnis, which means “meat.” There are other animals that eat nothing but grass or plants. They are called herbivores, from herba, which means vegetation. Imagine taking a lion, who is a carnivore, and placing a bundle of hay or a trough of oats before him. He will not eat the hay or oats. Why not? It is not because he is physically or naturally unable to eat them. Physically, he could munch on the oats and swallow them. But he does not and will not, because it is not in his nature to eat this kind of food. Moreover, if we were to ask why he will not eat the herbivore’s meal, and if the lion could answer, he would say, “I can’t eat this food, because I hate it. I will only eat meat.” Now think of the verse that says, “Taste and see that the LORD is good” (Ps. 34:8), or of Jesus saying, “I am the living bread that came down from heaven. If anyone eats of this bread, he will live forever” (John 6:51). Why won’t a sinful person “taste and see that the Lord is good” or feed upon Jesus as “the living bread”? To use the lion’s words, it is because he “hates” such food. The sinner will not come to Christ because he does not want to. Deep in his heart he hates Christ and what he stands for. It is not because he cannot come naturally or physically (pp. 85-86).
Apart from Him we can do nothing.
This is the first and most important secret of fruitfulness in partnering with Jesus in His greater works mission of the gospel.