(From a short project that I worked on recently)
There are several reasons that I can think of for shelter staff to be disappointed, frustrated or even infuriated by adopters or potential adopters, or people surrendering animals to the shelter.
There are people who consider pets to be disposable and replaceable. The link to petrescuereport.com (below) describes how pet owners surrender their animals to shelters before going on vacations, or before having large parties or family events. The shelters become overwhelmed with surrenders on a seasonal basis, while the former owners are off enjoying themselves.
Another point of frustration are the complaints from some potential adopters about the adoption process itself. I’ve witnessed customers at our shelter who objected to the questionnaire and interview process, and considered them to be burdensome and unreasonable. They seem to consider animals to be something that can be picked off a shelf, like a box of cereal, and aren’t open to the idea that the shelter staff are there to match them with the right pet. Admittedly, some private rescues have very high hurdles that adopters must cross in order to take home a pet (as seen in the below Slate article), but most shelters are simply asking common-sense questions.
The biggest source of frustration has to be with people who simply lie. They will turn in a pet, claiming that it is a stray in order to avoid paying a surrender fee. They will lie about the reasons that a pet is being turned in, its age, its health (possibly out of guilt), and any behavior or temperament problems. Adopters will lie about their family status, lease agreements, their living situation. Dealing with this will inevitably wear on shelter staff.