That is, if you are startled you aren't just, oh, how surprising! No, this is more of a, jump up out of your seat, move involuntarily, make noise, heart races, etc. sort of thing. (Training a startle reflex to do something really useful is core to some martial arts training.)
Lots of things can startle a person. A loud or unexpected noise or sensation, for example, might make you jump and cause your heart to race, even if it is not, objectively, frightening once you know what it is. This morning, when the van came to pick up A., I had lost track of time and was startled by the horn honking. It wasn't just the horn -- it was the oh, shit, I didn't get A. out to the driveway and now I need to make sure she has on her shoes and jacket and backpack and pause the TV show and so forth.
If you are very attached to a person (again, there's a technical meaning here, in which your autonomic nervous system and their autonomic nervous system become synchronous, at least when you are breathing the same air. When they are calm, you feel calm. When they are agitated, it is hard not to become agitated as well. Happiness, sadness, irritation -- all become contagious in the context of this kind of relationship), and that person startles, you might startle, too, even if you didn't have the same experience they did. So, for example, if a (HEY TRIGGER WARNING FOR ARACHNOPHOBES HERE) spider drops on them and startles them and doesn't touch you, you might still jump out of your chair in a panic because they did.
I know perfectly well that my daughter is that attached to me. But I'm still always kind of amazed when my startle gets reflected in her. The horn honked, I jumped up, and she jumped up and starts describing her feelings of surprise and being scared. And she doesn't even know _why_ she's suddenly surprised and scared, so I'm explaining to her what happened as I pause the TV show and get her jacket and shuffle her on out the door.
Parenting is so ... odd.