This is one of the reasons why when I end up waging war with someone else (usually after they have attacked me first) it's usually to the death - because they NEVER surrender, or accept a ceasefire, not even when I have three full-stack armies outside the walls of the shell of their last pathetic city (whose buildings my assassins have virtually destroyed).

I have been playing as England and allied with Spain so that I could focus on France and Denmark. I tried to make peace with Denmark but that wasn't working. I was allied with Spain for a LONG time, talking to them frequently and giving them map info, 500 florins and offering to attack the Moors for them which raised my relation status to Perfect. After wiping out France and holding the east border I started working on the middle east, Egypt and Africa. Many turns went by and I kept talking to Spain keeping my relations good/outstanding/perfect by giving them about 500 cash nearly every turn. I held Tolouise and Bordeaux and never made any agressive moves towards Spain. We're both at war with the Moors and he's fighting them around Corduba and has plenty of room to expand south. I'm only sending merchants to Timbuktu and only have Tripoli and the old Carthage city (forgot the name). A dozen or so turns later I see a stack of Spainish troops cross over into my lands. I send my diplomat to talk to him and give him 500 florins, map info and an offer to attack the moors all as a gift and he happily accepts and says how much he loves me. Our relations are "perfect"! The next turn he heads straight for Bordeaux and puts in under siege! I reloaded the last saved game and backed up a turn and get the Pope to call a crusade to take Corduba figuring that might distract his attention. Again I go and offer him 500 florins, map info, attack on the moors and he happily accepts. But as soon as I hit next turn he heads straight for Bordeaux again! Not even calling a crusade in a territory right next to him distracts him. Diplomacy is totally irrelevant.
This has just happened to me too. As Hungary I had perfect relations and alliances with most powers - I had a long-standing marriange alliance with Poland and Denmark, while I also had alliances with the Pope, Sicily, the HRE, England, France, Portugal, Spain and Russia (all on perfect), while I also had perfect relations and an alliance with Turkey (I wanted to secure my eastern flank while I eliminated the Byzantines). I had outstanding relations with Egypt, the Moors and Spain, although I didn't have alliances with them.

I could understand the Turks breaking their alliance against me as a jihad was called on Constantinople - after I bloodied their noses by destroying two full stack jihad armies outside Nicaea they sued for peace as our relations were still "reasonable" at that point. This seemed to be democracy which was logical, reasonable and based on geopolitical realities.

But what was really wierd was when a Spanish general with about 4 units (2 catapults, a jinete and a spear unit) landed on Ajaccio which I had taken from the Venetians some time ago, and attacked me for no reason at all! I had an alliance with them, and the status of our relations were perfect. Why? What did they have to gain? Even worse they refuse to accept any ceasefire, so I've been in a 10 year war with them where both of us are too far to attack the other. This didn't make any sense at all to me.

Another example was when, after my princess married the Danish king, I accepted a request from the Danes to help them against the HRE. I attacked from the east while they attacked from the north - I took Nuremburg after some heavy fighting. Then I saw a full Danish stack coming south from Hamburg - I thought it was headed to Staufen, but no - it headed to Nuremburg and the clowns attacked me! This is a nation that I had a perfect alliance with, had a marriage alliance and was helping in a war against the HRE. In addition, Denmark was ALSO at war with France and England, so it wasn't in their best interests to get embroiled in another war, with a strong and trusted ally (they only had about 3 provinces all up, and if they wanted to expand they could have done so to the north as Oslo and Stockholm were rebel provinces). It didn't make sense...