What is the current leading scientific explanation for why people dream?

Answer: During sleep, the brain sorts emotional memories and recent experiences.

During sleep, the eyes twitch and create the pictures seen in dreams. — Not quite. This mistakes the eye movements of REM sleep for the cause of dream images. The eyes do twitch during REM, and that is a normal part of the sleep stage, but the twitching is a side effect of brain activity rather than the source of the story. Dream content still comes from brain activity that draws on memories and feelings.

During sleep, the brain replays the day unchanged and adds nothing new. — Not quite. This treats dreaming as a literal replay of the day. Sleep does reactivate recent experiences, but dreams blend and reshape them instead of preserving them unchanged. That reshaping helps explain why a dream can include familiar places and people in a story that does not match waking life.

During sleep, the brain sorts emotional memories and recent experiences. ✓ — Correct! This is the leading explanation, though researchers still debate how much of dreaming it accounts for. During sleep, brain regions that process what we see and feel stay active and work over things learned and remembered from the previous days. Because the sleeping brain does not consciously pick one memory, different memories can merge in a spontaneous, unsystematic way. That merging is why dreams can feel familiar yet illogical.