Unimportant Thoughts - Nightmares

I was just wondering: why do we have nightmares? I know the<s> basics of </s>scientific answer, that our subconscious keeps our memories and fears and puts <s>it</s> them all together. (it must be wrong but o.k.). I was thinking <s>of</s> about it because <s>this week</s> I had nightmares every night this week! <s>And</s> I also had them in the afternoon, because I sleep after lunch. <s>And</s> I just can't understand why. I've tried to write a journal of each dream, but I'm lazy hahaha.