Why are dreams sometimes able to tell the future?

Answer 1

Favourite answer

Often that's because your subconscious picks up bits of information that you don't consciously notice, and can draw a conclusion about the future based on that.

For example, there is a couple that everyone thinks is very happy together, but you have dream that they broke up.  This happened because your subconscious noticed suspicious body language, the way they talked to each other, etc. that were red flags predicting the breakup. 

Answer 2

Without knowing exactly what you're referring to, no explanation is possible. Research dejávu and precognitive dreams.