Well the reason we rejoice in sufferings is because God said so. He said "In everything give thanks, for this is the will of God in Christ concerning you. That's in 1 Thessalonians I am pretty certain - yes - it's 1 Thessalonians 5:18
You see, Jesus suffered and died for us, so being Christians means that when things happen to us in this world which is inevitable in everyone's life - we can LOOK for the positive in the situation, and thank God for that.
Yes we still grieve when we lose a family member in death (especially in tragedy not just natural deaths from natural causes) we grieve when we lose our parents, grand parents even though that could be natural causes in old age; we get distressed over ordinary hardships in life such as lack of finances or any kind of tragedy you can imagine such as flood, fire, famine, tornado etc., I can't go into everything that happens but we all know everyone gets hardships of all kind.
Job was the example in the Bible who, when he lost his children his servants his cattle (which was his financial stability and food etc and he lost absolutely EVERYTHING EXCEPT his wife), he still remained resolute that God was on the throne and he wouldn't give God the blame.
The wife wanted Job to curse God and die, but his response in a nutshell was such as this - "Don't be so foolish wife, God gives and God takes away, blessed be the Name of the Lord" and the Bible is specific in that Job never sinned with His lips.
You see Job wasn't aware of the (spiritual) battle which was going on in the spiritual realm between God and the Devil but God was RELYING on JOB to stay positive and still love God even in all his despair and heartache at the loss of his children etc.
Christians are not promised an easy road. We are walking on the NARROW, DIFFICULT Way to eternal life.
The broad road to destruction is the EASY ROAD. That road leads to Hell's torment for eternity, so you don't want to be on that road.
Christians are mocked, persecuted for our faith - martyrs are actually KILLED as Jesus was for their faith so if we are only persecuted and mocked and if we only have to keep a positive attitude and keep loving and trusting God in our tribulations we are fortunate.
Jesus said, "Through MUCH TRIBULATION we shall enter the kingdom.
So all that God is saying is, "In this world things will go wrong and you WILL be persecuted and get sickness and tragedies in your life but don't give up. Keep worshipping and trusting Me even in the sad or bad situations because I will never let you down, I am with you always"
Therefore, if whatever you are going through is a sort of test so that God can show Satan how much we still love God in those bad times, we will be in eternity and have a wonderful future with God and Christ. Keep positive and stand up no matter what happens and things will not seem as bad as they are and you will have God on your side always. He loves us with an EVERLASTING LOVE that will never let us go and never let us down.
Don't forget that when the disciples were actually BEATEN FOR CHRIST< they went away stinging and hurting no doubt but they REJOICED and SANG PRAISES TO GOD that they could be so privileged as to be required to suffer for His Name. They were DELIGHTED THAT IT HAPPENED.