I hope Sirius constantly turned into a dog to get out of arguments with James, because it would mean that James was left with the following options:
- Being known as the crazy man who is arguing with a dog
- Rough-housing, and being known as the man who is mean to dogs
- Submitting to Sirius’s literal puppy-dog eyes, and losing almost every argument they have from the age of 15 onward
The fourth option is to turn into a deer and continue the argument.
Hogwarts student: *walks in on a deer and dog barking at each other*
Hogwarts student:
Hogwarts student: why does this keep happening