Sometimes they're sleepy. Sometimes they want more attention, sometimes less. Sometimes they're over-stimulated, and sometimes they're bored. Occasionally they are even sick or in pain. Usually, parents can find ways that work to soothe their babies when they cry.
Different babies soothe in different ways.
Some are easier to soothe than others. This is not always the case, however. Some babies cry for excessively long times and are particularly difficult to console. Parents differ in their tolerance for crying. This has led to definitions of Infant Colic for research purposes that depend on a crying time greater than four hours per day.