Idk if it's been mentioned, but before "This Version" of the manga came out, there was a different version that didn't get very far in. In THAT version, he wandered into the mountain by accident and then got chased in further by a bipedal fish thing and a swashbuckler looking humanoid cat (Think Puss-In-Boots, but make it appear more derpy). They proceeded to hunt him for fun mocking and harassing him daily. The bridge, which was the only way to escape the mountain and go back home was destroyed by the two creatures leaving him completely stranded and at their, and other monster's mercy. So he decided since he was stuck there anyways, might as well train and get stronger. Eventually, killing the two stalkers and escaping the mountain the opposite direction he entered.