Some suggestions;

When photographing pets, it's best to get down to their level for the good shots. Another trick is to run them around a bit to get them worn out so their tongue is hanging out or they're at least "smiling"

Last thing to watch is your background, the grain in the wood all points "away" from the dog to the the rug which now overly dominates the picture and draws the eye away his face. In this case you could have "hidden" the rug and a lot of the grain by doing a closeup on just his face. By cutting out his body and a lot of the background this would become very dramatic.

The nice thing about modern cameras is that they take shots at such a high resolution that you can still accomplish this in an image editor and not loose any quality.

This is a decent shot, but if you follow the above tricks it'll go to an awesome shot.

Serious? Proud perhaps!.a good image and like hamellr has stated,could be improved.....a good attempt and I can see why you overlooked cropping ,given that pooch's persuasive powers..........

Assertive - in my opinion - as much as a dog could possibly be LOL
Great portrait.
Yours SAM