He's not really protecting his old master's grave. In actuality, he's trying to protect you by preventing you from going into the Abyss...where his master sacrificed himself to protect him. Its a truly tragic story and a very sad fight. The DLC gives a bit more background on it.
But yeah, Dark Souls is a super immersive game when you let yourself get swept up in its unique storytelling method. Dark Souls 2 is not as good as the first in that regard, but is MUCH more fun to play (mechanically it is superior).