Just why do we have dreams at night, and wake up only to remember the bad ones and not the sweet ones?

I’ve been having dreams every night these days, which also means to say that my sleep at night wasn’t a sleep at all with such an active mind.

I know I had a sweet dream this morning, I know who are the people involved, I know that the process of the incident is very sweet (with some weird situations here and there). What I cannot remember of the dream is the exact details of how and why it happened. But still, 所谓日有所思夜有所梦. One never fails to dream about what one pins for in reality.

Been having headache since I woke up this morning. =(