...and cry as they are saying, "Why do I feel this way? My life is perfect. I can't think of any reason to be depressed."

It just is. It tends to run in families, and doesn't need to make sense. It generally takes some kind of trigger, but you never know what that trigger may be. Frequently it is a physiologically stressful situation that may be bad (illness, death in the family, loss of a job, financial stress) or good (getting married, birth of a child, a new job, a new home, moving to a new location). Either way, the stress triggers the depression, and too often we think that there has to be a good explanation as to why. This is why so many missionaries are hit with it for the first time on their mission. First time away from home, living independently, the pressure of full time service, etc. They aren't weak. They are human, and now they have to cope with some incredibly hard things essentially by themselves. No parents to support them, a companion that they have known for just a few weeks/months. No familiar teachers, bishops, youth leaders, doctors. Yes, they have Heavenly Father, but at this point in their lives, they likely are just starting to get to know Him, and still feel all alone.

I have it in my family. My mother suffered silently with it for her entire youth, then through her adulthood. When my sister and I both left on our mission on the same day, she fell into a deep abyss that she couldn't crawl out of. It was at this time that she first sought help, and finally understood what she had been feeling her entire life. Of my four sisters, two have it and two don't (that I'm aware of). The oldest is in Utah, and is a trying-to-recover alcoholic. Fortunately, I have been spared up to this point (age 48), but I'm not naive enough to think that I will never be affected. Hopefully I can recognize it if it happens.

My oldest daughter really struggles with depression and anxiety. She tried to go on a mission, but was overwhelmed with the anxiety and had to come home. It crushed her, but she is self aware enough to know that she didn't do anything wrong, but just bears a cross that she has to cope with on a daily basis. It remains to be seen what will happen with the other kids.

I really admire Tanner for bringing this up. He didn't need to, but by doing so, he gives hope to a lot of others who have suffered silently, and continue to do so. This is especially good for the LDS faith, because too often we correlate feeling bad with being bad, and that just isn't the case with clinical depression.