Honestly, I'd have to side with Death here. Unless I've misunderstood the story thus far, it seems as if Death and Life once were in a romantic relationship that ended in a bad break up - circumstances still unknown, my theory possibly it being over War or at least that having been a part of it. Anyway, even after they broke up, Death couldn't get over Life. He still loved him, hence Love's sympathetic reaction in 'Understand Me - 16.'

Then, we have Life, who is full of mixed feelings about Death and decided to finally confront him and begin a new friendship, which in turn, unintentionally led Death to believe that Life had forgiven him. Not all the way as to jump back into a relationship with him, but at least to give the chance to start new.

So, from Death's perspective, he thought that Life had put everything that had happened behind him. That he'd been forgiven, to an extent. So to find out that Life had forgotten, either by self-infliction, trauma or other, must be heart-shattering. To look at the person he loves (Life) and know that everything that has happened is only by the effect that Life doesn't remember - that their history has been erased. I honestly would have reacted the same.