This has more to do with serotonin levels in the brain than anything else. When the brain is stimulated (throughout the day or whilst in REM sleep), serotonin levels drop. Since serotonin is found as both a somatic hormone and neurotransmitter, low levels can propagate a variety of effects within the body, such as tiredness and weakness, or even effects on the opposite extreme, such as stimulation. If levels of serotonin get really out of balance, it can cause psychosis.
Depressed people have less active brains throughout the day (likely do to a more subdued lifestyle) than happy or manic people, and thus their serotonin levels don't have time to drop enough. Thus, when they sleep (if their depression doesn't affect their sleep) they have a larger bank of serotonin that needs to be used up, and thus they dream more.
Why serotonin is needed to dream isn't exactly entirely clear (asides from the fact that an active brain uses it up). Perhaps it is used in the production of endogeneous dimethyl tryptamine within the hypothalamus, the only known endogenous (produced within the body) psychedelic. It is found to be released whilst in REM sleep, and is thought to be responsible for the varied and realistic nature of dreams, along with other phenomenon, including OBEs and NDEs.
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