How did dreams become aspirations?

Why have the dreams we have in our sleep become equivalent to our waking aspirations, such as Dr. Martin Luther King’s “I have a dream” speech?

I have a lot of dreams that I recall. Of 30 dreams this year, only one might be considered wish fulfillment and even that ends on a weird note. I expect most people share that experience: Dreams are strange, unsettling, and seldom take us to a place we want to go.

So how did dreams, which don’t fulfill our wishes, become associated with waking thoughts about our wishes?