"At The Vet's" joke

Three Labrador Retrievers, a chocolate, a yellow and a black, were sitting next to each other in the waiting room at the vet's office.
The black Lab turned to the yellow Lab and asked, "Why are you here?"
"Well, I'm a pisser," the yellow Lab replied. "I pee on everything. The bed, the sofa, the carpet."
"So, what are they going to do to you?" asked the black Lab.
"They're going to put me on Prozac. My master heard it's a miracle drug that supposed to cure everything," explained the yellow Lab.
The black Lab thought about this for a few moments, then turned to the chocolate Lab and asked, "Why are you here?"
"I'm a digger," replied the chocolate Lab. "I dig into everything. The flower beds, the lawn, even the cushions on the chairs and the sofa."
"What are they going to do to stop you from digging?" inquired the black Lab.
"They're going to put me on Prozac. Apparently, it's supposed to be some sort of miracle drug," the chocolate Lab replied.
As the black Lab sat silently and pondered both answers, the yellow Lab turned to him and asked, "What about you? Why are you here?"
"Oh, well, I'm a humper," the black Lab declared. "I hump anything and everything. Trees, the mailman, the furniture. Just the other day, my mistress had just gotten out of the shower and I humped her too!"
"Wow! Are they going to put you on Prozac too?" the chocolate Lab interjected.
"No, I'm here to get my nails trimmed!" replied the black Lab.