The thing about small dogs is, well...they are small! Which means their belly hair is close to the ground and can get dirt and debris in it and easily be soiled or tangled. This means that cleaning, trimming and/or brushing your small dog's belly may be a frequent necessity. Also, because small dogs are close to the ground they are liable to pick up parasites like ticks and fleas that live in underbrush and dirt or sandy area and that especially like to reside on the underbelly and under limbs of your dog.
Regular grooming of your small dog's belly will bring pesky parasites to light that can be treated quickly, and regular cleaning and brushing to prevent mats from forming will prevent moisture being trapped on your dog's belly that can cause a host of unpleasant skin conditions.
Most dogs like having their belly rubbed, but not always having it groomed! The belly is a sensitive area, and your small dog may not like having hair brushed in this area if it gets pulled or appreciate having a clipper or scissors used to trim belly hair. To make belly grooming more comfortable for your small dog, you can incorporate grooming with petting and belly rubs. Start with your dog on your lap, pet and rub his belly, use a rubber mitt to get him used to a different sensation, and then move to brushes, combs, clippers etc. Keeping belly grooming sessions short and combining grooming with petting and belly rubs will usually get your small dog acclimated to having his belly groomed. He will soon be happily displaying his underside for regular grooming.