The Modern Book of Fortune states a "generally held belief", that I've never heard before, that "dreams go by opposites". It doesn't really describe what this means except that, while we spend the day with our conscious mind comparing and contrasting things, the unconscious mind has no power of contrast so when we dream of black it may really be white. Which may just be an overcomplicated was of saying that dreams don't make sense, but the time I dreamed of a burning fish tank I was fully aware that the water shouldn't be on fire.
Man has been interpreting dreams since he's been having them, and many ancient civilisations believed they were divine messages. These days we tend to attribute them to physical causes (illness, stress, too much cheese before bedtime), even if there are plenty of books out there that will tell you that dreaming of your teeth falling out means the end of a friendship.
I've personally found that dreams are usually my brain trying to tell me something I'm not hearing while awake. Nothing to do with the future, but often the solution to a problem I may not even realise I'm having. If I'm anxious and I know it, I dream about being chased. If I start dreaming about toilets - specifically being unable to find one that isn't overflowing, or where the cubicle actually has a door - it means I need to take stock of what's bugging me, because something is and I'm not thinking about it.
Essentially, dreams are the subconscious talking. Sometimes it talks rubbish, sometimes it just tells a funny story, but I've found that by paying attention, it can provide an insight into waking life.