The word surprise by its very nature is a non progressive verb (in most cases) and therefore you will see it used in passive forms. Those forms will be followed by at or by - in those cases either the preson or thing does the action.Originally Posted by carla guaraldi
I am so surprised at/by her behavior.
I am really surprised at what I have seen/by what I have seen.
by/at expresses the same idea. No difference.