Why is it that someone's cuntitude jumps up about 20pts as soon as they buy a house?

I've just been faced with a conversation about:
Should rachel sell her house now that she's married?
No, she can rent it out and will be happy because rent is more than mortgage.
Karen plans to have her house split into two when she has children.
Rachel thinks a special kids bathroom would be good, with smaller showers.
Both think this would add value.
ADD VALUE!!!

Rant over.