Job. Abraham. Others. God either put into their lives something they greatly feared, or asked them to do something that - I believe - they greatly feared.
One of the parts on the path to God is that He will ask you to do something you do not want. It appears that it is something that is fearful to you. Abraham was going to be offered up as a human sacrifice. It is only natural that he should fear not only having such a thing done to him, but having it done to his beloved son. He had apparently already lost one son when Sarah kicked out the boy and his mother. He may have been fearful of losing his other son. Job plainly says that the thing he greatly feared had come upon him.
A friend and I were talking about Abraham's sacrifice. She pointed out that God wants us to face our fears and overcome them. That is the point of asking us to do the one thing we most don't want to do.
It is not so we can take a stand against God and say, "No. I'm going to be an adult here, and I will not do what you ask." (There is a thought process going around that Abraham lost his exaltation because he obeyed the command to offer up Isaac. Such a notion contradicts scripture.)
Neither is it to feed God's pride, or to show us and Him that we will bend to Him at all costs (though bending to Him at all costs is part of the process).
It is God helping us to over come all fear, all hesitation: Fear of what others will think about us or do to us. Fear of losing everything. Fear that your only son will not only be killed, but will be killed as a sacrifice.
Do we trust God? Do we trust that what He asks us to do is for our own benefit? Truly? Deep inside? Does God love us? Is He selfless in that love? Can He be trusted with all that we are, have, and believe?
It is a process of learning to trust God completely. When He asks for that one thing that we fear most, that one thing that we are most reluctant to put upon the altar, we are learning several things:
God always comes first. Not us. Not our families. Not our religion. Not our money nor our job. Not our reputation.
We learn to overcome all fear. After that experience, we fear nothing.
We learn to be selfless. Those who become eligible to be as God, must be as selfless as He is.
I'm sure there are many other things to be learned from this, however, this is all I have right now. Isn't scripture study incredibly interesting? Isn't there so much to learn and gain from looking deeply into what is there, and asking God to open up to us what we do not know?