WriYe - Character Relationships

How hard is it for you to create character relationships? Do you pre-plan them or do you end up letting them develop as the novel goes on? Have your characters betrayed you and paired up with someone you didn't expect?
My relationships in stories tend to be pre-planned in the first plotting stage. For example, in Thief I knew that Dawn would have a boyfriend that she'd leave behind when she ran away and he'd come after her. I knew Lois's parents were married happily and that she didn't have a boyfriend. I knew that both Callahan sisters were flirts but that there were no serious relationships going on (and it would take a very dedicated man to have a relationship with Morgan when she turns into a man herself every so often...)

I've never had a character do something completely unexpected with regards to relationships, although occasionally they will say something which then makes me think they could do with a pairing somewhere.

Give us the story of how your favorite written characters got together and what makes their relationships strongest.
I honestly can't think of any! I don't really write romance, and the relationships that are in my stories tend to be pre-existing without any back story to them. I suppose if I had to choose, I'd say Lois's parents in Witness. I have no idea how they got together, but they complement each other very well - she's soft and comforting and he's practical and full of useful (useless?!) advice. They also foster kids and still manage to raise Lois as a fairly normal teenager. I think I really should flesh out their characters and back story more in a short story some time...

Bonus: Give us a picture of your planning process when it comes to characters!

This is a kitten randomly booping a dog on the nose. My character relationship planning is often rather like this - it just suddenly happens. I'll suddenly think of a character and then realise they need a significant other, or evil ex, or a crush, or someone else to make them more of a fully rounded character.