I. God allows it for your good

 A. It is a "messenger of Satan"--cf Job 1:12

 B. God makes it work for good--Rom 8:28

 II. He wants you to pray about it--Lk 18:1

 A. He may not remove it--text, vv8-9

 B. He will give you grace to bear it--v9

 C. He will make you better through it--Isa 40:31, Isa 41:10, Eph 3:16

 III. It is a work of God to boast about

 A. It makes you rely more on Christ--Heb 11:33-34

 B. When you do, your weakness becomes strength--2 Cor 13:4

Albert Barnes writes:

Learn:
 (1) That a Christian never loses anything by suffering and affliction. If he may obtain the favor of Christ by his trials, he is a gainer. The favor of  the Redeemer is more than a compensation for all that we endure in his cause.
 (2) The Christian is a gainer by trial. I never knew a Christian that was not ultimately benefited by trials. I never knew one who did not find that he had gained much that was valuable to him in scenes of affliction. I do not know that I have found one who would be willing to exchange the advantages he has gained in affliction for all that the most uninterrupted prosperity and the highest honors that the world could give would impart.
 (3) Learn to bear trials with joy. They are good for us. They develop some of the most lovely traits of character. They injure no one if they are properly received. And a Christian should rejoice that he may obtain what he does obtain in affliction, cost what it may. It is worth more than it costs; and when we come to die, the things that we shall have most occasion to thank God for will be our afflictions. And, oh, if they are the means of raising us to a higher seat in heaven, and placing us nearer the Redeemer there, who will not rejoice in his trials?