During the stressful part of my life
Usually when I'm concerned about certain situations which results are strongly emotional for me, such as an important exam, a special meeting, a date with a beautiful woman, may be a particular buying, danger, finishing a job, etc. I become kind of obsessed with those matters and it's very difficult to lose attention on them. Therefore I'm constantly thinking about them during all day. Eventually, I end up dreaming about what I want (or don't want) to happen.
I hated it, but now, I realize it could be very useful.