Prepare for Bad News
One thing has become very clear to me about dreams, and that is they are mostly bad news. An examination of Bible dreams reveal this truth:
Of course not all dreams are bad news: a wine steward received good news that he was going to be released from prison, Genesis 40:9-13 and Joseph was told he would be greatly honored. Genesis 37:5-9 However, even in Joseph's case, his good dream did not materialize until many years later, and only after first passing through some terrible tragedies. If you are going to take dream interpretation seriously, then be prepared for bad news. Dreams are not some sort of "sleep television," come to entertain us while we sleep. God has more serious purposes than that, for His desire is to shape us into the image of His Son, Colossians 3:1-17 and that means delivering us from our own sinful tendencies which lead to our eventual harm and destruction.
And that is why dreams are mostly warnings- it is God protecting us from ourselves.