When I feel myself closing down, and wishing the interaction to be over so I can go and spend time with my favorite companion, I try to force myself to reopen my mind and remember that the dog in front of me owes me nothing, and deserves everything.
Love Without Attachment
Another benefit to the practice of metta in shelters for me, is that it has forced me to focus on non-attachment. Love without attachment is really important in Buddhism, because we believe that attachment causes the world to be full of suffering, and fails to recognize that everything in the universe is transient. Love without attachment means not clinging, or expecting anything from those we love, but being able to let go.
There is a constant flow of animals through the shelter I volunteer at, which is of course a good thing - it means we're doing our job well. This high turnover can be emotionally draining; as soon as you build a bond, they leave with their forever family, their room is filled by someone else and you're faced with building a whole new bond. It can feel like a repeating pattern. After this happened a few times, always leaving me feeling sad, I realized that seeing a pattern is itself a mistake and a failure of metta - to the dogs, this is all new of course, and each dog is an individual with its own point of perception.