Pain and frustration are part and parcel of the life of a believer. Paul is the best example for this. However, nothing affects a believer beyond the purview of God. Is there not a child that is not admonished and corrected by a father? However, there might to many reasons.

One, God might want to test our faith in Him so that we ourselves might know how strong we are in the Lord. (God knows it already).

Two, He might want us to come closer to Him in our difficulties - not go away from Him. He wants us to trust Him more and more that on our own abilities and our reserves.

Three, He might want us to humble ourselves so that we don't lift ourselves up because of some special revelations He gives us or some special power He bestows upon us.

God allows difficult times – even calamities – in our lives in varied durations and severities. God even allows Satan to continuously pound a believer, like how He did in St. Paul’s life - there was given to me a thorn in the flesh, the messenger of Satan to buffet me. – 2 Cor 12:7. The Lord did not remove this thorn in the flesh; Paul gives a reason for it – lest I should be exalted above measure – 1 Cor 12:7. Pain and sickness need not always be because of sin; it could also be because God wants to keep a believer from sin, though for a season. A believer can rest assured that nothing happens in life that is beyond the purview of the Lord. The Lord allowed Satan to lay his hand on Job’s possessions, children and later his body too. For we have not a high priest which cannot be touched with the feeling of our infirmities; but was in all points tempted like as we are, yet without sin. – Heb 4:15. For in that He Himself hath suffered being tempted, He is able to succour them that are tempted. – Heb 2:18. But we also need to note that even Satan cannot snatch away the life of a believer from the safe hands of the Lord. We are under his protecting and caring wings in Whom we have our refuge. He sends trials and tests so that our faith in Him would increase over the course of our lifetime, as in the case of Abraham. While everything might seem to go out of one’s control, God assures that He has everything in His control. Praise the Lord. Glory awaits a believer who endures sufferings and tribulations. If so be that we suffer with him (Christ), that we may be also glorified together. – Rom 8:17.