I mean, as a piece of literature, it's not stellar. It's not particularly well-written, and you can see through a lot of the plot devices. For example, Bubba is a pretty blatant self-insert, and Matt is less a character and more so someone for Bubba to talk to. I'm not quite sure why this is a book about dogs, and not just an essay on positive psychology. Most of the book is Bubba talking about positive psychology anyway. I think I agree with most of what was said, but there were better ways to say it. It's not exactly a nuanced take.
I do believe that Jon Gordon, the author, had good intents upon writing this, but it misses on a lot of the nuance of living life. It shows that being positive won't always fix everything, but it shows that it'll lead to happiness somewhere down the line, which isn't always applicable to real life. Positivity isn't a switch to happiness, but it sort of presents it as such.
It has a plot. I can't say much more.
If you're looking for an enjoyable read, probably don't spring for this book. If you're looking for something to read to your kids, it's a fun afternoon that is easily digestible.