Why do I dream when light sleeping?

Actually you remember your dreams more readily when you are in a light sleep and awaken quickly. You kind of catch yourself in the process of dreaming or falling into the dream stage of sleep. Also they have done many, many tests/studies in sleep labs and ppl go through stages of sleep and dreaming all night long, not just during the initial falling asleep period. We are all living, sleeping, breathing, awaking in our own body time period.

You're crashing into REM

It is not true that we only dream during REM sleep. Dreams can occur during any sleep stage.