First of all, small dogs are, of course, petite. They weigh less. You can pick them up, put them in your purse, have them come on an airplane with you, and you can cuddle with them in your bed without it taking up all the space from your significant other. They don't eat much compared to some of the bigger breeds, are easier to groom and care for, and are mostly cheaper when it comes time to take them to the vet. For instance, it costs less to spay or neuter a small dog than it does a large one. Dogs can be lovely bite-sized additions to any family unit.
If you live in an urban neighborhood, a small pet might be your best bet. People who live in apartments or houses with not much of a back or front lawn may do well to adopt a pup who doesn't need that much free time outside. You can also take the little doggies out with you into the city, where many big dogs may not be allowed: on the bus, in a restaurant, or even under your arm when you're shopping. Most small dog types outlive their more gargantuan relatives. Many small-dog breeds live upwards of 13 to 16 years, depending on the breed and from dog to dog. All in all, small dogs are cute and loving in a manner that other, bigger dogs just cannot match.
Of course, there's the other side of the coin, as well. Little dogs tend to yip and bark more. This can cause problems with neighbors and, while you may be able to teach your pup to be quiet, they tend to go right back to barking when you're not around. and can get into more trouble when it comes to sneaking out of their designated areas. It may feel odd to own a pet that, were you to step on it, you would kill it. Even though your pet may be small, it is not generally aware of that fact, and may get you into trouble by portraying boldness even in the face of certain death (e.g. other, bigger dogs). You may have to tug your little guy aside when big dogs come, lest they start a fight they will in no way win. You know what that means, right? That's right. No unleashed walks in public.
There's also the case of what many owners know as 'Small Dog Syndrome'. Because your pet is so small, it will be natural to coddle it. Some owners even treat the small animals like little children, resisting the notion of scolding the pups if they do something wrong. However, the dog needs to know you are in charge and it must abide by your rules. If you have a dog, even a small one, make sure that it knows you're the boss, or it may begin to exhibit signs of this symptom: it will growl if you approach its food or try to move it when it doesn't want to be moved, and may even snap and bite at the hand that feeds it.
But you love small dogs, so you're willing to make the sacrifice. Well, you may find them a bit difficult to housetrain. They are notorious for having 'accidents' in the house (or in your purse, if you're prone to putting them there). Dog training of all types can be difficult for your poor petite pup, as they just don't have it in them to learn.
Make sure to consider both sides of the story before adopting your small pup. They can be rewarding pets to people who live in urban areas, but you must be prepared to be a proper puppy parent when teaching and disciplining them. You will find them all the more loving for your effort.