Alright faving this one. You know the title would fit better if it where 2 guys LOL.
What is up Seiya? I mean she was locked in the shrine and the person told her whom ever realises her would be the one she would love and serve, but she just says well if you say so or was she a servant before so being one is just what she understands??
There can be a lot of explanations. One of the simpliest - she is away from shrine as long as she loves and serves her master. And inside the shrine is something bad - from agonizing pain to the simple eternal boredom.
Or another (just to show you the variety) is that Seiya was deeply brainwashed and her personality is locked deep inside. So the thing we see is just a mindless toy hoping to free itself.