In no particular order. And because "the world around them" is ALMOST right, but not quite.
  1. Other dogs
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    This sometimes includes each other.
  2. Vacuum cleaner
    Also DustBuster, but not on the same level.
  3. Children
  4. Wind
  5. Cardboard
  6. My husband reacting to a football game with fantasy ramifications for him.
    Whether those reactions are positive or negative is irrelevant.
  7. Dancing
  8. The doorbell
    And the fact that they are not permitted to go rip the doorbell ringer a new one...or at least shrilly freak out directly into his or her face.
  9. Standard iPhone noises
  10. Baths
  11. The time between when they think they should be served dinner and when they are actually served dinner
  12. Unexpected signs in the yard
  13. Bicyclists, joggers, and families out for a nice stroll through the neighborhood
  14. A lack of complete focus on them when they've deemed it to be cuddle time
    Put that phone down, or prepare to be swatted at, obnoxiously and repeatedly, until eye contact is established.
  15. (More to come, I'm sure.)