Some dogs are in kennels for a long time, and are over-looked by prospective adopters for a variety of reasons.
Many dogs are very excited to see visitors and become bouncy and noisy. A quick walk would show their true character.
The main reason dogs are not chosen is because of behavioural problems. Most dogs who are difficult have suffered abuse or bad handling in their past. It is not their fault and they deserve a second chance.
For an experienced person, willing to give a dog time, patience and understanding, this can be very rewarding. The bond between handler and dog will be very strong, and you will have a very loyal friend- a friend to be proud of.