Kosuke used to be a womanizer and that's why he moved away and started living alone in the woods, to have some time to himself and contemplate.

The film uses tiger and dog as opposite symbols, the dog being represented by Shiori as a person who has sex anyone they want and is open and free, the tiger isn't represented by any particular person but by the environment Kosuke lives in, he used to be a dog but now he's in search of his inner tiger, "The lord of the forest. It's not a wild dog, it's from a greater realm.", he says as he gazes upon the woods with wonder in his eyes.

Tigers are solitary animals but dogs are in packs, that's why at the end we hear multiple dog howls and one tiger roar. It makes sense from a storytelling point of view, there has to be more than one person for the sex to start! Shiori, the wild dog goes around and turns other people into dogs, after that fateful night we see all the involved characters go nuts and keep having sex.

At first Kosuke looks down on her and disrespects her by calling her a stray dog, a name she will later put on display as if she's proud of it. Maybe she wanted to help him get out of his shell or maybe she was just doing everything based on her dog instincts, either way she ended up helping.

The author views sex as a therapeutic method, it helped that girl get over her past trauma and find the love of her life, at the end Kosuke's hut in the woods is in ruins and his time there is over, he can now go back a new person, a dog. She even leaves a note for him, playfully asking him "Who's the dog?". The girl was the hero of the story all along, forcefully healing people one by one, going even as far as rejecting Kosuke at first just so they could wait and have a wild long sex during which his hut breaks down.

At last, the tiger is captured and confined, and the dogs can prevail, Shiori is elsewhere probably saving others. The dogs start howling and the tiger gets in one last disapproving roar in vain.