
As well as possible changes to your dog's temperament you have to remember that as the owner of the sire you will also be held legally responsible if the subsequent puppies turn out to have avoidable health issues, such as hereditary conditions, unless you can prove that your dog has been tested clear.
Some dogs don't change very much after they've been used at stud, just becoming a little more 'manly', but some become nightmares, leg-cocking indoors where previously they were clean, becoming inter-dog aggressive, much more likely to roam, mounting any dog or bitch whether or not in season, howling ... The trouble is you won't know how he'll react until after the deed is done.