We all do it; we startle. But what startles one may not do a thing to the next person or dog. I have three dogs as you know; both girls are pretty rock solid and Luke....................well he is not. He is about the most nervous dog we've had. He is not the most nervous dog I've met; far from it. He is not constantly looking over his shoulder, is not afraid of people or dogs; he's just jumpy. Luke does not like things that move on their own, door that blow, bags that blow, broom that slide down the wall and smash on the floor etc.
For Luke there is something mighty creepy about things that simply move on their own. He also is not a big fan of being stepped over; just try it and see. He flies up and out of position nearly knocking you to the ground; I have learned to let him fly before I step. I have actually tried to startle Tilley just to see how much it would take; in 12 years I've only startled her once. Jessie is starting to startle alot lately but I know that is because she is loosing both her hearing and her eye sight. Just imagine. I will often reach out to pet her and she startles; telling me that she didn't see my and coming towards her.
You can help dogs that startle alot by introducing them to as many things that might startle them small increments and work up. And like most behaviors the biggest help is your reaction; don't apologize, coddle or otherwise treat your dog like a baby. If you knocked a book onto the floor and it really startled your dog; say nothing bend down calmly and pick it up. Then go on with what you were doing.
You can actually create problems and cause issues that are already ingrained to become larger just by reacting. "Oh my gosh I'm sorry; did that scare you poor little guy?" I often slam doors because Luke has a real fear of doors slamming. He once had a seizure when he was very suddenly awoken from a deep sleep by a slamming door. This has left him with a fear of doors that move or blow in the wind.
Lead by example; your dog is watching.