I am absolutely terrified of spiders… I once came home at 5:30am after a night of… trouble making (this is a total lie, I was a huge nerd and was out playing Dungeons and Dragons with friends).  I parked my car in the garage and walked toward the door to get inside… until I stopped dead in my tracks there was a GIANT… okay big (quarter sized) wolf spider ON THE DOOR.  I panicked.  I started shaking and took 5 big steps backward.  I pondered for a moment… I knew our front door was locked… and our back door was locked… the only way in was this door…. but there was a spider there that I couldn’t even get close enough to to kill.  So what did I do?  I “manned up” and called my mommy.  Yep at 5:30am I called my mommy to open the door for me because of the spider.   Not my proudest moment…but… I could see no other option (I knew my mom would be waking up for work so it wasn’t THAT bad).
My mom came downstairs, looked at the spider… and said, “you’re fine… it’s big, it’s scary, but it’s not going to kill you.”  It was calm but kind.  Her reassurance didn’t make me fear the spiders any more or less… but it did soothe me (also made me feel better that she agreed the spider was scary enough to warrant my trepidation).
What does this story have to do with dog training?  Well, I hear time and time again that, “If a dog is acting fearful you shouldn’t tell him it’s going to be okay or pet him because you will reinforce the fear.”  It seems the ideal (from most of the people who have told me this) is that the dog sits with the feeling of fear/anxiety until something is resolved.  It is sad to watch a dog who is fearful be left to get through the scary moment with no guidance or reassurance from the handler–often I see the dogs looking to the handler, desperately trying to communicate their discomfort or get some feedback.  I want you to think of the thing that scares you most… what would reinforce your fear of that thing?  What would make you less fearful?
For me, my fear of spiders would absolutely be reinforced if I got bitten by a spider, had an allergic reaction, nearly died, then had a MASSIVE medical bill to pay because of the bite (melodramatic I know).  I have a fear of falling from heights (heights are okay if there’s no chance for falling)… probably the only things that would reinforce that fear would be falling from a height OR having some mean person tease me by pretending to push me off the ledge.  Having someone reassure me would make me neither more nor less scared, but would make me feel better in a given situation.  If a person were screaming and panicking, “OMG!!!  Are you okay!?!  You are fine, but get over here away from that thing!”  I would probably not feel very secure about the situation and may panic even more.
Calmly telling a fearful dog that “it’s okay” or trying to soothe an anxious dog isn’t going to reinforce the anxiety or fear.  Fear is more of an emotional state that is most often reinforced by the fear coming true (my being bitten by a spider or a germaphobe getting the flu or someone scared of clowns being ‘attacked’ by clowns in a “fun house”).   Soothing a fearful dog calmly is not going to reinforce the fear or reinforce the behaviors (many of which are not operant behaviors anyhow–in other words many of the behaviors done out of fear are not choices by the dog, they are reactions to a stimuli) but may help a dog get through the moment.
Now, I do think owners who overreact to their dog’s fearful response can make their dog think there was a reason to be fearful.  So a small dog who is fearful of some large object and the doting owner swoops in, picks up the dog, quickly moves it away from the object while coo-ing and fussing over the dog can be problematic.  It’s like when a parent sees a child fall; if the parent calmly says “stand up, you are fine” the kid gets up and keeps going, but if a parent rushes over in a panic and is frantically searching for the mortal wound, the child bursts into tears with added dramatic screams.
Don’t be afraid to calmly soothe or reassure a fearful or anxious dog if the situation warrants it, it isn’t going to “reinforce the fear.”  It won’t solve the problem in the big picture, but it may provide some comfort during the difficult moment for the dog.  If a dog is fearful or anxious, getting the heck out of dodge is the preferred solution (other than actually working on the fear issue) but if that isn’t an option, it will not hurt to calmly reassure the dog.