On the surface, the speaker is apologizing for bringing home a dog. But the "apology" is really a list of the dog's charms in disguise. He follows you around the attic, guards the top of the stairs, barks at the doorbell, walks calmly beside you, and grins with his whole mouth. Each of these is presented as a fault, yet each one sounds like exactly what a person might want in a companion.
The joke works through the gap between what the speaker says and what she clearly means. The "I'm sorry" repeats like a refrain, getting less believable each time. Even the line about dogs "notoriously" shining at programming is gently teasing. By the end, the complaints, such as the dog pulling you from work, keeping you home, and emptying your pocketbook, build to the real confession: this dog "compels you to love him." The mood is warm, playful, and affectionate, with a little mock-guilt for fun.