Best Friends

So, you know when people get married, they say they married their best friend? Why is that? My best friend is Tyler, who's a guy(if you didn't catch that) and I'm not going to marry him. Does that mean he's going to get replaced? Are we just friends until we find something bigger and better out there in the "real world"? See, I'm confused. I don't understand it. When will I understand it? Mom and Dad said they are best friends, but what about their other friends, who have been called "best friends"- I'm probably rambling about nothing, but I just didn't understand what it meant.