Why do people often think others are the same as them?

[krystal]

Sometimes when I'm browsing the questions on this website, I noticed that people who give answers think that because it took them a certain amount of time to do something, it'll take the asker that amount of time as well. I know what I typed sounds confusing, so let me use an example:
Mary wants to learn Japanese, so she starts with the katakana and hiragana. Her friend Mark, who is learning the language himself, tells her that it will take her months to learn it, because it took him months to learn it. Why does Mark automatically assume that Mary can't learn it faster than he did?

This can be applied to all areas of life, I think. Why do people assume their experiences will be the same for everyone else?