I've run amateur dream groups, where people get together and tell each other their dreams, and then each participant interprets the dream back to the dream-teller. One method was to start one's interpretation by saying, "If this were my dream..." This helps the interpreter relate what they are hearing to their own life, and anything they might say to the dream-teller is really about the interpreter and their own psyche. So you might try such a self-reflective stance if you ever get stuck having to listen to someone else's dream.

With that said, if I ever choose to tell a trusted friend about a compelling dream (in a non-dream-group context), I try to relate the story in about 20 seconds, just hitting the highlights, cause I know how you feel about having to listen to someone else's unsolicited dream story!

I think it's for the same reason that "It was all just a dream" is such a terrible plot device.

There is a meaninglessness inherent in the retelling of a dream. You know at the outset that it didn't actually happen, so why is it interesting?

Most people, even when being quite generous and patient, need to have some involvement in the story to remain interested. For me it's a similar annoyance as listening to someone's long vacation story or about a project idea they have that you know they won't actually start. But at least with a vacation story you can imagine yourself taking the vacation someday, but with a dream you can never go there or experience the same things so it's intensely boring.