I volunteer at a local animal shelter near my home and I can tell you that while it is incredibly gratifying, it can also be incredibly heartbreaking. These poor animals are often terrified or experience high levels of anxiety once they're brought in to the shelter, especially if they're surrenders from a family that for whatever reason can't keep them anymore.
I'll often spend a few hours in the shelter talking to the animals and cuddling with them, simply trying to get them to feel comfortable with their new surroundings, because not only does that make their time in the shelter more enjoyable but it also makes them more adoptable. You can imagine how scared some of them are after being brought in to a new environment and surrounded by other animals. Many of the animals in shelters undergo surgery once they're brought in, being spayed or neutered, and that can often add another level of anxiety.
One poor pup was having a hard time after surgery at the BARCS Animal Shelter in Baltimore, until one vet assistant stepped up to help the poor little one out.