The answer for me lies in God's purpose not only for the individual but for his purpose in the world.   I believe God healed me because he had a work for me to do that would be accomplished by my being healed.  Indeed, God healed me dramatically in an instant.  I have told my story over and over all around the world.  Many people have had their own faith deepened when they have heard what God did for me.   Because of my testimony people know that God heals. 

 I have known people whom God has not healed.  In fact, I have prayed for people but they were not healed.  Sometimes I have a sense as to why.  Perhaps the person has unforgiveness in their heart or bitterness from some experience.  Sometimes they really do not believe God can heal.  The Bible is clear that faith is believing God and that he does what he says he will do.  There are some times I have not known at all why someone was not healed.  In those cases, I have to accept that God is sovereign.  

One of the most encouraging things anyone who is praying for healing for themselves or someone else can do is look up the scriptures in the Bible on healing.   The gospel of Mark will really build one's faith when they read what Jesus Christ did as he went from place to place. 

As you read the scriptures, ask the Holy Spirit to teach you what they mean.  Expect God to speak to you through the scriptures, to open your spiritual insight and draw you into God's understanding about what you are praying about.