Is there any reason why I would NOT want to have a friend?

[RelVleDy]

I don't get it. I notice events give me the option of not being a friend with someone. Why would I NOT want someone as a friend? It's a flat +100 relationship bonus, which I'm pretty sure the A.I. never goes against you when they're your friend. Furthermore, I've noticed special favorable events fire if I have at least one friend. I'm not sure, but I might also get a bonus to assassinating them, since they relax their guard around me maybe?

Is there something I'm missing here?

 

[Gnorrosch]

You can duel rivals if you have the the war focus, thus it might be handy to rival someone who has a low martial score and either threatens you or from whom you will inherit.

Otherwise, that is one of the typical AI trap decisions that plague this game. There are several decisions where one choice is clearly superior, but the AI chooses randomly - you will see plenty NPC rivals from that decision.

 

[joos]

Maybe you are shy, proud and envious

 

[Thorkel the Tall]

Possibly a RP option? You might want a rivalry with the neighbouring duke, rather than be friends with him (especially if you played an honest character, you might not approve of using a friendship ties to take over his domain).

 

[Dragatus]

Rebelling against your liege and fighting him for his title also requires you to have a bad opinion of him.

So there are a few special cases where it's better to not be friends. But they're relatively rare.