[Echidna]

This is a good portrait of your lab and whilst most the comments are valid, they do not take much away from the photo.

As for the question you asked about composition and your lab being centre, don't worry! After all it is a portrait and regardless if your subject is human or canine they can be centred and take up all or most of the frame.

Too many ppl can't get over the "thirds" rule (it is really a guideline - as it doesn't need to be complied to always).

By the way love the eyes looking up - lab eyes always get me

Enjoy.