1. The effort of house training them from 'new'
2. The embarrassment of them humping anything with a leg
3. Having to clean up its waste (shudder)
4. Having to take it out in all weathers and at times inconvenient to you
5. The cost of feeding it
6. No chance of impromptu weekends/last minute holidays away due to having to find a kennel or tolerant friend to look after poochy
7. Vet's fees
8. It might be a barker
9. Its farts
10. Even if all of the above could be tolerated, one day, it will die and point ten is the main reason I won't have a dog.