Things cannot always be pink and fluffy...
When you look at a photo, the image is made up of light and dark. Different tones create a picture which your eyes can see; the world is like a photo. There are good things and bad things, good things are light, and bad things are dark. If the photo was all light, you'd never see the image becuase you'd never be able to tell the difference between the tones, you would see nothing. It is the same with the world, if everything was good, and nothing was ever bad, you would have no understanding of what 'bad' was, and you would feel nothing. 'Bad' would be a completely alien concept to you. Which means with nothing to compare against, you would loose all understanding of 'good'. Good would become 'normal' and you would never appreciate the good things in life.
The world is made up of opposites, light and dark, day and night, sun and moon, good and evil, you must have a balance to make the world right.