what triggers dream when it keep on come and go?

i have one dream or should call it nightmare(?) that keep on repeating for the past 15 years. I had it again last night. what actually triggers dream? i though we dream when we think about something too much... but honestly for the past recent years this thing never bother me as much as it did before... so what triggers dream when it keep on come and go and yet i dun give a damn about that matter anymore now. Many things are in my control and i control it well so far.why the dream never dissappear when the rain has washed away all the pain of yesterday?