Ren's got a chip on his shoulder and something to prove.
He doesn't know who he was or what he was about before he got here, and you can bet your ass he's mad about it. He also doesn't know where he lost a chunk of his ear or where his tail is, and he's mad about that, too. Here's a couple other things that make Renwick angry:
It would seem he is part dog.
Conflicting instincts.
The texture of sand.
His abundance of fleas.
The prevalent aroma of wolf urine.
His own hind paw when he scratches himself, sometimes.
Weather, be it hot, cold, or pleasant.