There are many factors involved : Stress level determines dreams, if you are stressed out about exams , you see all sorts of stressful dreams 2.Also depends on what you ate before you slept. But some dreams are memorable. I donot know the scientific reasons but i have been somewhat brainwashed by old nepali sayings . for example. 1 If you see yourself dying , they say you will live longer 2 if you see blood , it is a good omen, 3 if you see snake ( nag ) , it is considered auspicious 4. if you see god and goddess, more auspicious 5 if you see teeth falling , then some one from your family will die ( i dreamt that my upper tooth came out, my grandmother passed away the next mornig) 6. If you see shit , you will earn more money and the list goes on. Guys feel free to add more.