One thing to keep in mind here, regarding the "spidey-sense" passive use of Sense Motive. Most characters don't have ranks in SM. So even their "always on" skill check is about a 10 (assuming Take 10). A character with even just 1 rank in Bluff (or no ranks in the skill, but a 12 Charisma) can Take 10 and get a subtle deception past most people without any real effort.
Most normal/common people in the Real World also wouldn't be considered to have ranks in SM. And they are the dupes for all those who have above average Charisma and/or ranks in Bluff.
I think what throws some DMs, and Players too, is when there is a PC like my 1st-level paladin -- Wisdom 12, Charisma 14, Skill Focus (Diplomacy), Negotiator = Diplomacy +11, Sense Motive +7.
Even though he is only 1st level, he is very well (self) trained in social interaction. Even a crafty con-man is going to have a hard time getting anything over on this guy. This is something that was irking a couple of my fellow Players in our game. They saw their characters as sly bluffers -- 14-16 Charisma, 4 ranks in Bluff = +6-+7. They saw their characters as able to get a deception or misdirection past just about anyone not very much higher level than they. But then they have my paladin in their group, and they find him hard to bluff. He's very socially intuitive, and sensitive to verbal and body language. In effect, he crimps their "style" (but only in regard to bluffing *him* personally).
My character is the seasoned street cop to their wily street conman (and woman).
This also makes it more difficult for a DM to have an NPC mislead a party and make an adventure "more interesting". A PC can sweet talk an NPC in giving up info or helping him, without actually having to do something like go on a little side trek adventure or prove himself.
I am not a strong speaker, nor very sensitive in social interaction in Real Life. Even if the DM was the best actor in the world, I still could not play my paladin by straight role play. I have to roll the dice and let the game mechanics work for me. And the game mechanics say my character is *very* well spoken, and very sensitive to social interaction.
Quasqueton