Why will small dogs do this?

Trixie

Whenever there is any space between my back and the couch Trixie immediately claims it as her own.  Skooching in between and settling in.  

As long as she doesn't land on one of my herniated discs I don't mind her sitting there.  (Although I have concerns of squishing her)  

Does anyone know why they do this?  I have heard of this but never seen it before. Let me know if you have any ideas.