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I think that your title is redundant...


Nevertheless, there is certainly a reason for it. Isn't there an area to see why your liege is angry with you? I haven't played CK in a while.
 
I asked a similar question early in January, "why did my liege jump me?" and the answer I was given is that the AI will declare war on you if they have a claim, sooner or later, no matter how loyal you are or how good the relations are otherwise between you and your liege. I don't know if this is true but it is certainly consistent with what I saw. It's one of many details that I hope will be better in CK2.
 
I got fat stupid and happy with my first game or two where it did not happen. Now, it seems to happen in every game.

The AI only seems to DOW you when you and his demesne share a common border (and he is not pre-occupied with something else) so try to prevent that.
 
It's a little hard when he is your liege. I'll admit not impossible, but still fairly hard.


Ah, nope.

What usually happens is that the AI declares war on his dukes/princes, and you help, and get some provinces to make your duchy prettier.
You just give out the provinces which border his own demense to some counts and that's it.