Disclaimer: I claim no copyright ownership of Mouri Kogorou or the Detective Conan series.
He didn't need some fancy shrink to tell him that he was depressed. He could read the signs. Self-medicating with alcohol and chain-smoking… estranged wife… gambling addiction… inability to get himself outside most days, his world shrunken to his comfortable chair, his TV, and his desk. Heck, his daughter was more like his care-taker.
Failure. Worthless. He had to depend on the money Eri sent him every month. He was a ball and chain dragging them all down.
And then a little kid was added to his responsibilities. The kid was nothing but trouble. That first night, he was almost beaten to death by a kidnapper. No wonder his parents didn't want him.
Another person unwanted by the world.
And freaky intelligent. The kid tried to hide it, and he let the kid pretend to be normal. Just like he did, when he scraped his office clean and pretended that he was accomplishing something with his useless life.
Maybe the two of them could do something about their situation.
Then, like he'd been possessed by some kind Kami, he started solving cases. No just the normal ones, really difficult ones. Then, he was famous.
The fame was intoxicating. Being adored by people felt fantastic.
But then, it'd be over, and he'd be back at home, with his comfortable chair, his TV, and his desk.