For males, for instance, these are the considerations:

Pro’s on getting your male dog sterilized

• You’ll reduce his aggressiveness by castrating him. Since testosterone is responsible for a level of aggressiveness, neutered male dogs are less likely to fight with other dogs over food, space or female dogs as dominance descends.
• You won’t have to suffer your dog disobeying your call because a female dog is in heat and your pooch can’t control himself over it.
• For surveillance dogs, this is particularly useful as no matter how hard your dog is trained, a female in heat will attract his attention and get him away of his duty. Same thing for all service dogs apply
• You won’t face the wrath of a female dog’s owner that has gotten their dog’s unwanted litter on your account
• You can sleep tight at night knowing that you will not be responsible for unwanted puppies whose life will be most likely miserable, according to statistics.

Con’s on getting your male dog sterilized

• Your male dog can gain some weight. This is both due to the reduction of testosterone, which will slow down their metabolism, but also because they will get more lazy and for you it’s going to be more difficult to get them to exercise. Don’t worry, healthy food and will to power will counter-count this 
• It’s been reported by some dog owners a big change in the pooch’s motivation in general. If you have a very active dog, it might come as a blessing, but a lot of owners say they wouldn’t have preferred this change in their dog’s character
• You’ll need your dog to go through a (minor) surgery, and therefore there are the common associated risks like infection, not proper cicatrization, etc. But this is less so with males than female dogs
• If your dog is a beautiful and excellent individual, who in time demonstrates to have wonderful genes, you cannot change your mind in order to have descendants from him (although maybe in some years there will be the option to freeze your dog’s sperm, who knows?)
• Watch out with your puppy’s breed: some breeds are specially prone to tumours after sterilized. This is the case of some Afgan hounds and Weimeraners, although studies haven’t been able to confirm this entirely. Check with your vet before this decision

Now for the ladies, the list may be a bit longer:

Pro’s on getting your female dog sterilized

• You will totally remove the possibility of uterus and ovary cancer, false pregnancies or milk secretion. You’ll reduce importantly breast cancer (depending on the moment the sterilisation occurs, the sooner the better). The recommended time is just before the first heat
• Some female dogs get really grumpy when in heat, so you won’t have to suffer your dog disobeying your call because a male dog is near or her escapes hunting for a mate
• You won’t face the economic and moral tragedy of an unwanted litter. Looking out for a home for these poor little ones can be tricky and difficult, and you will wind up with that in your conscience (if you have one)
• You can sleep tight knowing that even if your little girl is in the backyard by herself, no one will get her pregnant
• You don’t have to clean after your female dog when in heat (aproximately 5-10 days, depending on the individual, can last the bleeding). Your poor pooch will be the whole day licking her private parts. Besides, most female dogs get a change in their temperament during these days (no wonder!)

Con’s on getting your female dog sterilized

• Your female dog can gain some weight. For girls this happens more often than for buys. Again, good food, exercise and will to power will be required
• You’ll need your dog to go through a major surgery, that requires general anesthesia and and the post-operatory can be really hard on her. You will need to stay with her at least the following 3-4 days. there are the common risks associated to this procedure like infection, not proper cicatrization, etc. A really big scar will always be on your pooch’s belly. Some dogs even require hormonal treatment afterwards.
• If your dog is a beautiful and excellent individual, who in time demonstrates to have wonderful characteristics, you cannot change your mind (although maybe in some years there will be the option to freeze your dog’s ovules, who knows)
• If spayed at a wrong age (too early) your dog will grow unevenly and may have other complications with liver and kidneys due to bad developing
• If sterilized too old, your dog will increase the risk of urinary incontinence, changes in behaviour and temperament, hair loss, displasia and bone-ligaments diseases
• Watch out with your puppy’s breed: some breeds are specially prone to cancer after spayed. Investigate thoroughly and check with your vet before this decision
• If your female dog is a bit grumpy or aggressive, spaying her might make things worse. Watch out for this! Studies have found that the presence of oestrogen helps reducing aggression, so sterilisation in female dogs, unlike male dogs, don’t help with this problem. Check with your vet and your trainer/behaviourist before taking the decision.