Because our Prophet Muhammad (pbuh) says so:
Hadith - Muslim, Narrated 'Aisha
Gabriel made a promise with Allah's Messenger to come at a definite hour; that hour came but he did not visit him. And there was in his hand (in the hand of Allah's Apostle) a staff. He threw it from his hand and said: Never has Allah or His messengers (angels) ever broken their promise. Then he cast a glance (and by chance) found a puppy under his cot and said: Aisha, when did this dog enter here ? She said: By Allah, I don't know. He then commanded and it was turned out. Then Gabriel came and Allah's Messenger said to him: You promised me and I waited for you, but you did not come, whereupon he said: It was the dog in your house which prevented me (to come), for we (angels) do not enter a house in which there is a dog or a picture.
Allah's Apostle said, "Whoever keeps a dog, one Qirat of the reward of his good deeds is deducted daily, unless the dog is used for guarding a farm or cattle." Abu Huraira (in another narration) said from the Prophet, "unless it is used for guarding sheep or farms, or for hunting." Narrated Abu Hazim from Abu Huraira: The Prophet said, "A dog for guarding cattle or for hunting."
Let him knows his deduction of good deeds (with exception in that Hadith).