Surprised at vs. Surprised by vs. Surprised to (Surprised + Infinitive)

The past participle surprised can be used with infinitives (verb forms beginning with to):

surprised to discover that he had left

One can be surprised by many things:

surprised by the attention she received

surprised by the sudden change in direction

In addition, one can be surprised at a person (or a person’s behavior, attitude, etc.) for not meeting your expectations. Usually this connotes an attitude of disappointment:

surprised at your disrespectful attitude

surprised at his sudden reluctance to help

Note that in the last two examples above, surprised by would also be acceptable, but it would not convey the connotation of disappointment as clearly; it would convey only the fact of the surprise.