Guidance in Dogs
Question :

I want some guidance regarding keeping dogs. Can we keep dogs in the house, considering that they serve as guides to the blind, they safeguard homes and are a good friend?

Answer :

You can keep them in the house, but they are najis. You should purify whatever part of your body or clothes comes in contact with a dog via dampness before offering the prayer.

Dogs
Question :

Why is the dog najis in Islam?

Answer :

Islam is a divine religion that Almighty Allah revealed to guard the welfare of people. Almighty Allah knows the reality of things and what benefits His slaves, so He set laws accordingly for them to observe. Since Islam is the seal of religions, these laws are for until the end of this world. Because of corruption, badness and evilness in certain things or acts, He commanded us to refrain from them and made them prohibited. Similarly, because of benefits and goodness in certain other things, He ordered us to perform them and made them obligatory on us. Some of these laws are general and apply to all people, all the time, and anywhere.
The ability of the human being is limited, so is unable to find out and understand the reasons behind the Sharia of Almighty Allah. Sometimes Allah Almighty has taught us through the Holy Prophet and the Imams (peace be upon them), the wisdom behind certain few rulings. We must obey His laws by refraining from the prohibited acts and undertaking the obligatory acts, even if we do not know the reason behind each and every one of them.
The najasah of the dog is one of the rulings about which we do not know the wisdom behind.