I believe it's about being too enmeshed, not having enough detachment, differentiation. I'm not sure whether the theory is that the partner is sooo close and familiar that there's not enough oxygen to ignite a spark, or that protecting the sexual space is the only way to preserve any individual identity. Or life gets in the way and resentments build up... I'd believe any or all of the above.
I'm sure JJ is fascinated by all this, too, but I'm going to end the hijack here.
