In Spencer W. Kimball, Faith Precedes the Miracle [1972], 98

 Why are we here on earth?

Here's the commentary entitled: "What is the purpose of trials?" : 

"Consider Elder Orson F. Whitney’s counsel about the purpose of trials: “No pain that we suffer, no trial that we experience is wasted. It ministers to our education, to the development of such qualities as patience, faith, fortitude and humility. All that we suffer and all that we endure, especially when we endure it patiently, builds up our characters, purifies our hearts, expands our souls, and makes us more tender and charitable, more worthy to be called the children of God … and it is through sorrow and suffering, toil and tribulation, that we gain the education that we come here to acquire and which will make us more like our Father and Mother in heaven” (in Spencer W. Kimball, Faith Precedes the Miracle [1972], 98)."

I do know that no pain that we suffer, no trial that we experience is wasted. Our pain shapes us to become our better versions--more like our Father and Mother in heaven. We came here on earth to obtain happiness and joy. To achieve this, going through sorrow and suffering, toil, and tribulation is necessary. Opposition brings out the best in us. Through experiencing what is the opposite of happiness and joy, we're able to figure out what brings us happiness and joy. And then we're able to choose happiness and joy over sorrow and misery.