According to Web MD, a cat's excessive crying or meowing can be caused by illness, hunger, or even simply wanting attention. Each cat is different and may vocalize for different reasons, so it is best to find the cause and narrow down the reason for their communications.
A cat that is ill may meow seemingly for no reason and may have an underlying condition that only a veterinarian can diagnose. Some breeds of cat are just more vocal than others. For instance, Siamese breeds "talk" to their owners seemingly just to hold a conversation.
Sometimes a cat simply greets its owner with a meowing noise, displaying its happiness at seeing him or her. A change in a cat's living environment can also spark bouts of noise as it express its displeasure at the changes. New people, objects, or moving to a new home can cause a cat to become unhappy enough to make a lot of noise.
Cats that spend long periods alone may require more attention than an owner can provide, requiring the owner to hire a pet sitter. Cats that are getting old may sometimes experience mental confusion and may require medicine to get them to quiet down.