The mirror shows what you want, so the idea is a “good” person like canon Harry would see themselves finding the stone (which allows them to actually find the stone), whereas a “bad” person would see themselves drinking elixir of youth or making gold. Harry would presumable see himself providing everyone with elixir of youth, which means that he would not see himself finding the stone, which means he could not find the stone.
The mirror shows what you want, so the idea is a “good” person like canon Harry would see themselves finding the stone (which allows them to actually find the stone), whereas a “bad” person would see themselves drinking elixir of youth or making gold. Harry would presumable see himself providing everyone with elixir of youth, which means that he would not see himself finding the stone, which means he could not find the stone.