I was discussing this with my dad the other night. He said "If you've never spent the whole morning out in the snow for long enough that you're completely freezing, you never truly appreciate the feeling of a warm blanket and hot chocolate. You might use a warm blanket or drink hot chocolate, but you don't truly experience it until you've had it in that situation."
That really made sense to me. If everything was constantly sunshine and rainbows (or warm blankets and hot chocolate) you wouldn't really love it the way you can love it when you've experienced the opposite.
And that's why sometimes you need to have a down day. Some moments, you need to just allow yourself to really experience the emotions you are experiencing. Choosing to be happy is good. I mean, you're going to be happy, of course it's good. But nobody genuinely feels happy all the time. Sometimes, you need to be honest with yourself.
And really, I think it's because of opposition that life is so wonderful.