When you are little, your nightmares are of monsters.
You dream of being chased by these creatures, haunted by them.
However, when you wake up, these monsters don't exist.
When you are older, your nightmares are of reality.
Real things that are happening, people you know and care about.
Although when you wake up, the nightmare isn't true.
The "monsters-of-reality" remain.
(and it's coming to get you.)