Sebek admires you for your strength and diligence, in his mind if he beats you, he could be strong enough to be worthy in Malleus' eyes. So as kids Sebek would always follow you around studying how you train and how to beat you. Other people around him could notice his admiration slowly turn to obsession real quick over the years. When in NRC he noticed how you've been ignoring him and hanging out with other humans. He's not jealous! He just thinks someone like you should be hanging out with him