Why do people always want what you dont have? Why do they never want what you do have?

In the shop and in the kennel I find that to be the exasperating case. If I have green, blue and yellow collars , you can bet they want pink or red...If I have Chihuahua pups in black and tan, they want brown or spoted,,if I have females, they want males...If I have help in the shop, no one needs their dog groomed, but let me be there alone,,,,oh boy! every one and his brother has a tripple matted, double dirty, rabies infested Chow that absolutely has to have his once a lifetime shave down...and "could you leave the hair about an inch long? I don't want to see skin!"