He was assigned as your knight, you find it difficult to accept because everyone keeps treating you like a child...

"Now, Link is here to protect you. So try to hold your temper."

Your father, the king assigned Link to protect you as you are next to heir the throne and nothing is allowed to happen to you. And therefore he chose the hylian champion, wielder of the sword that seals the darkness to keep you safe.

Although, the two of you don't get along well...