This is probably the newbiest shit ever, but something I haven't really considered before. Why is it that in a mix everything is recorded in -dbs? I've always just taken it for granted and not really thought about it until now, so what's the actual reason behind this?

There's a lot of write about this. Google "producing headroom" in google and you'll find a lot of articles.

Usually I sit around -8 to -10 and take the loudest part, which is the kick and build the rest around it.

Such as; http://blog.landr.com/7-tricks-to-create-headroom-and-how-it-will-save-your/

I believe the logical reason is this:
When you make a track, you have no idea about how loud it will be played back by the listener. But you need some kind of relative measure, so they came up with this idea to take a maximum amplitude for the signal distribution (+- 1 volt I think, 16/24/32 bits digitally), name it 0db and base every measuremet out of it. (Thus logically you go down.) That maximum is in the end controlled by a listener's volume knob. (When he sets the volume to 100db, your -5db peaking track will be 95db loud in reality.)

tl;dr: a DAW has no way of representing a signal that goes higher than 0dB, thus creating distortion

yeah basically that. you dont really need to know that much, basically 0db is the maximum db
and any volume that's not filled below 0db is called headroom

Are you asking why about the unit "dB" and why sound is measured with it, and the difference between dBFS & dB SPL? - Decibel - Wikipedia

or...

Are you asking why there's a negative sign by the number specifically in the context of digital audio?
This post is very easy to follow: Sound Design - stackexchange: What is 0 dB in digital audio?