First, because relationships are more complex than this. Having friends doesn't mean the same for everyone, and even one person will not think the same way about two different friends. There are nuances to every relationship that depend on the characters, and that make it unique.
There is something else that makes every relationship complex, and that is not as often reflected in writing: two characters in a relationship will not think the same way about each other.
The typical example of this is the unrequited love interest. Character A loves Character B with a fiery passion, but Character B couldn't care less.
There are so many other ways this could be exploited, however! I think it's worth it to consider the world from another perspective than the hero's, and see what the surrounding cast think of him, and of each other.
- Character A could think Character B is a rival, and feel the need to prove himself the better of the two, while Character B is only seeking a friend.
- The hero can have a sidekick, and believe him to be super loyal, whereas the sidekick spends his time wishing he was with another knight.
- Character A might confide anything in Character B, but Character B seeks comfort elsewhere when he needs it.
- Character A can think of B as nothing but a colleague, whereas Character B thinks of A as a close friend.
If this isn't fun, I'm not sure what is!