it's not that they "know" you or "understand you" more than anyone else. they're not magical, or better than any of the rest.
you just want them to, and so that's who you tell your shit to. you want them to know your favorite kind of cookie, your favorite color, your childhood memories. your dreams.
so, that's what that is. don't be fooled into thinking it's different. it's not different. they are.