Playing!!!
The one on the left looks like its either mugging for the camera or startled at getting caught.
I believe the sheltie is the aggressor. Ears are up, teeth are bared.
The other dog dog is in more of a submissive posture, ears are pulled back, teeth not bared.
Would be a fun photo to pull out the dogs and paste in a different scene.
Additional accessories needed for photographer - long lens, bucket of water or two and stout stick.
There was a time when I quite liked dogs, Trina, then I started walking with a bridge camera strung around my neck. After being set upon by several teeth baring demented mutts whose adoring owners assured me they were only playing, or usually very friendly or somesuch I now don't give any dog the benefit of the doubt and either walk on the other side of the path or roadway or prepare for evasive action.