15 parts Complete Marriage. The state becoming involved in your relationships to humans. The way a Time Lord says "I love you".
And then there's regeneration. When your body, mind, interests change. When apples go to yogurt to fish fingers and custard.
When sharing house with someone, you get used to all their little quirks. All the small things they do. The way he runs his hand through his hair, her smile, all the seemingly inconsequential mannerisms like posture and how they sit on the sofa.
All of the things you love about a person suddenly change, leaving you with a new spouse. It's fine, though, because you have the doubt. Do they still love me? Do I still love them? It is, after all, a completely different guy from the one you've spent three hundred years doing DIY with. What if this one doesn't know how to hold a hammer correctly? Can you really bare to be around a person who doesn't love DIY?
So, as you can imagine, regeneration causes problems.
