why do i dream about things i barely think about in waking life?

I find that if i intently try to dream about a particular subject by running it through my head all day so it is deeply engrained in my brain, i will never dream about it.
However, when i think about something that is just a passing thought that i rarely pay full attention to for more than a few seconds, i always dream about it.
Is it because the subject matter of our dreams favor our subconscious thought over our conscious thoughts?
Everytime i think about dreaming, i think of that song by the Everly Brothers that goes "dreeeeeeeeeam, dream dream dreeeeeeam" to associate lucidity with a song.
I've been doing that for weeks now and haven't had it play in my dream, yet i think about some stupid game i used to play for about 5 minutes during the day and dream that i am in that videogame.
Kinda frustrating.

Yeah, it happens to me also. I guess our subconscious likes to create a dream from the everyday things in our life, the stuff we think is very normal.
Maybe when you were playing that game you were in a state of thought that it was implemented on your subconscious and, perhaps, when you think of that song you are just thinking and singing about that, not giving it's full attention.

Sometimes this happens to me too, except the times, when I had stressful day or something special going on. E.g yesterday i was worried about the marks in my school, and i got a dream where my chemistry teacher screams at me, because I denied to go to the chemistry competition :D

Yes. The subconscious takes center stage, while dreaming. It is much more common for you to dream about concepts that are abstract to you (which go on in the back of your mind), than it is to be dreaming about something (in its exact form) that is mainly in your conscious/immediate mind.