I think the worst thing that we do in relationships is...
The Mother of All F*ck-Ups:
Partner Assumption
Dialogue might go like this:
Man: "Hi! I like you lots! See, I got you this huge expensive sapphire engagement ring!"
Woman: "Oh, that's... nice. Thank you."
Man: "Whoa... what? You don't like it?"
Woman: "But I... I kinda wanted a diamond."
Man: "...a diamond?"
Woman: "They're the de facto engagement ring stone."
Man: "You said you didn't like diamonds because people in Africa get butchered for them."
Woman: "Yeah."
Man: "You were IN Africa for 6 months and saw it."
Woman: "Yeaaah."
Man: "You still want diamond?"
Woman: "Girls LIKE diamonds!"
Man: "People suffered and / or died for the shit!"
Woman: "Ugh! You never think about ME!"
Man: "But... but... I THOUGHT you were a people-loving hippie!"
...
I think the problem is that we get SO comfortable with our partner that we fail to do those "maintenance queries" every so often that eventually our asses are skipping around sideways in conversation like a too-scratched LP record. We know the channel we THOUGHT were we on with them about Topic A, B, C... but we didn't really check after a week, a month, a year. As the time line goes along... you build on your notion of them instead of what they actually do / say / feel. It almost always seems to culminate in some gigundo Pompeii-esque incident that utilizes the relationship no-nos of "you always" and "you never" to the max.
...
Assumption: I know I screwed that up. Thanks a lot, Global War on Terrorism.