There are various reasons:
It was commonly believed at the time that Jesus lived, that sickness and suffering were the result of sin. Jesus contradicted that idea. In Luke I3: 10-17, after healing the crippled woman, Jesus said: (v,16), 'This woman... had been kept prisoner by Satan for eighteen long years.'
In Eph. 6:12 Paul reminds us that 'our fight is not against human foes, but against Satanic powers, against the authorities and potentates of this dark world, against the superhuman forces of evil in the heavens.'
Paul knew something about this personally. In 2 Cor.12:7-10, he says: "I was given a sharp pain in my body, which came as Satan's messenger to bruise me. Three times I begged the Lord to rid me of it, but his answer was: 'My grace is all you need; power comes to its full strength in weakness.'"
Query: Do you know any Christian who, although suffering, is triumphant in faith? Pray for him/her just now.
Cause and effect: God is not only a God of love but a God of law. The laws the Creator established for his universe must be obeyed or a penalty will follow.
Illus. The much-loved American woman, Joni Eareckson-Tada, often asked herself why God should allow her to be paralysed from the neck down. The reason was obvious. As a teenager she dived head-first into shallow water - something no swimmer should ever do without checking the depth - and impacted her neck and spine. It was a tragic case of Cause and effect, with life-long suffering to follow. But like Paul, Joni was given spiritual grace and strength to triumph, and has become one of today’s most effective evangelists, particularly to young people.
Imposed suffering: Wars, persecutions, terrorism, muggings, etc.
Natural disasters such as floods, droughts, earthquakes, hurricanes.
Hereditary factors e.g. Haemophilia - blood weakness - passed from mothers to sons.
Self-inflicted illness. e.g. lung cancer through smoking, anorexia by girls too anxious to slim, drug-addiction...
Infection from contact with germ or virus carriers: e,g, AIDS.
Wrong emotions. Anger, resentment, envy, etc. all cause disharmony in both spirit and body, leading to ill-health.
Finally: we live in an imperfect world, on a contaminated planet spoilt by man. There is air-pollution from car exhausts and factories, water pollution by waste products being poured into rivers and seas, and food pollution through chemical fertilisers and additives in foods. It is no wonder that we are sometimes sick:
DISCUSS: What life-style should we adopt to live as healthily as possible, remembering that our body is 'a shrine of the indwelling Holy Spirit'? (1 Cor. 6:19)