Why does everyone hate me?

Er no. For one, the Fatimid sultan in 1066 has a long reign bonus already. Also, married couples only really hate each other if their traits clash. A little bit of gold and then religious conversion will smooth things.

But, it is true that some of the game's vassal setup at the start mean that you as the liege is holding onto your vassals' de jure vassals. In 1066, I know Philip I of France is holding one of William's de jure vassals, and 2 of another duke's. Before you unpause the game, check for such stuff. You cannot do anything about being foreigners, short reign and trait clashes, but you can do something about this.

What really hurts is having low diplomacy and negative prestige at the start. Add on the dishonorables you may incur while trying to off some vassals and it can be a slippery slope. I think what really does it in though for rookie players is that marraige at the beginning where they go for stats and ignore the huge prestige hits for marrying the talented lowborn.