Why do we dream some nights and not others?

Perchance to dream: We actually dream every night, and typically several times a night. REM (dreaming) sleep occurs about every 90 minutes during sleep. But memory doesn't work during sleep, so the vast majority of dreams are immediately forgotten. If instead we awaken during the dream, either because the dream is too disturbing, or because outside stimuli awaken us, then the memory of the dream can be constructed.