The creapiest suggestion I've heard (from here, can't remember whom) is that they need the blood of the living as their lubricant.
Or perhaps their power source (as in an internal combustion engine's power source is gas/petrol) is life essense, which it needs to keep running.
Or it could have been formed by some dark pact, and can only repair damage by transfering it onto some other being.
So, there are plenty of alternatives if one is willing to picture the mechanics in a manner that makes sense for a sentient, energy dependent automatom.
As a side note, in my own campaign I intend to use the vampire rules for someone (not a warforged) raised from the dead by an NPC lich - basically, they exist as normal, but are unable to heal themselves without stealing health from someone else.