Some cats are more vocal than others, but if you have a cat who won't stop meowing — especially if this is new behavior — it's worth looking into. Cats have many weird behaviors that are innocuous, but if they're meowing excessively, it could mean they need something from you, or possibly that they're in pain.
It probably comes as no surprise that cats sometimes meow when they're hungry. To some degree, this behavior is normal; your cat might meow a moderate amount while you're opening up their food can. (Mine certainly does.) However, if they suddenly start meowing considerably more at mealtime, or throughout the day, take them to the vet to make sure something hasn't changed with their health.
Another unsurprising reason your cat might be meowing is for attention. If your cat is bored or lonely, they might meow so you pay attention to them. If this happens, don't give in when they meow, but rather provide more entertainment throughout the day. Buy or make them some stimulating toys, and make sure you dedicate time every day to playing with them.
Stressed cats are likely to voice their stress loudly. Has a big change recently come into your (and your cat's) life, such as a new baby or a new pet? Cats are sensitive to change, and can become stressed in new situations. If you suspect that your cat is stressed because of a recent change, give them lots of love, attention and quiet one-on-one time to help ease their anxiety.
A cat that's meowing excessively might be in pain. It's always a good idea to take your cat to the vet first to rule out this possibility.
Just like people, cats can experience confusion as they age. If you have an older cat that's started meowing more, take them to the vet. Vets can prescribe medications that help cats suffering from confusion or disorientation.
Desire to breed (if they aren't fixed).
If your cat isn't fixed, they may yowl when they're in heat (for females) or when they smell a female in heat (for males). If your cat isn't fixed, take them to the vet to get fixed as soon as possible.
What to do when your cat is meowing.
Always take your cat to the vet when there's a marked change in behavior, like sudden excessive meowing. The vet will be able to rule out possibilities like illness and disorientation, and will help get you on the road to curing what ails your kitty.
What not to do when your cat is meowing.
Don't punish your cat for meowing. It won't work, and it will make your cat distrust you. Don't give in, either. If your cat is meowing for food or attention and they receive it from you, they'll learn that this technique works, and then you'll be trapped in a cycle of meowing for the rest of your life together.
When in doubt, always check with your vet first, and show your cat lots of love and patience.