"a calm sea does not make a skilled sailor"
You LEARN how to deal with a storm by only being in a storm, and by going through that strom and surving it, you become a stronger individual for it.
But If all you had was CALM waters, HOW DO YOU LEARN?
how do you become stronger?
how do you become skilled?
for a sailor - in order to navigate in the worst of conditions, he/she must be able to tackle stormy weather, but if the sea is always calm without storms, how will they learn? they are novices, amatuers, immature. This will leave them at a disadvantage when the real hurricane comes, and they have no experience and have no idea what to do to survive.
This is the same for the Christian walk, In order for you to be ready for the real hurricanes that you will have to overcome, God walks you through storms, small storms that build you up and train you, As you get more and more experienced, the storms get stronger and stronger, till that day comes when the hurricane hits, and you are ready for it. -(Inspired by R.M)