The dreamer of particular dreams is dreaming dream-perceptions (imagining them to be real) unaware he is dreaming.
He does not know that he is dreaming since he is not aware that he is being dreamt!
Only the ultimate dreamer (universal consciousness) knows it is dreaming all dreams.
Out of its freedom and sense of play, consciousness chooses to forget that it is dreaming as it freely identifies with the dreamer of particular dreams.

The dreamt dreamer of particular dreams cannot know it is dreaming since knowingness does not belong to it. Knowingness belongs to the one and only real dreamer of all dreams: universal consciousness.
All dreams are temporary and fleeting. You, the real You, the ultimate dreamer is untouched by any of it.