They're hungry. Babies cry when they are hungry, and if it's been 90 minutes or more since their last feeding, there's a good chance that hunger is behind those cries.
They have a dirty diaper. A dirty diaper can be uncomfortable, and can lead to a rash if left on your child for too long.
They're tired. An exhausted child is often cranky, and they can wear themselves out even more with the crying.
They're overfed. Too much food — or being fed too frequently — can lead to indigestion and a fussy baby.
They've had stimulants. Caffeine from coffee, tea, chocolate, or other sources may lead to fussiness.
They've been startled. Sudden loud noises near a baby may set them off. Babies often have no idea what's going on around them, and that can be unsettling.
They're going through a growth spurt. Babies are constantly growing, but they may have spurts of growth as well. The La Leche League of Canada explains that during those times, the baby may be hungrier and fussier than usual.
They're uncomfortable. There may be some crying if a baby gets uncomfortably bound up in clothing or blankets. They can also be picky about temperature, and don't like being too hot or too cold.
They have colic. Babies that cry for more than three hours per day may have colic. The U.S. National Library of Medicine explains that as many as 20 percent of babies may have colic, and that it usually starts around three weeks of age and resolves by the time the baby is 12 weeks old. Colicky babies usually cry about the same time every day — often in the evening — and they have moments of contentment and happiness when they are not crying.
They're in pain. Keep an eye out for possible illnesses or injuries that may cause pain. Earaches or sores in the mouth can be a source of distress for your little one. When serious pain is an issue, the baby will likely be irritable even when not crying.