Why is my baby crying so much?

CNS immaturity: Evidence suggests that most babies cry "too much" in the first 3 months because they can't calm themselves once they become "sensory overloaded". In fact, the icd (international classification of diseases) has recently recognized this and designates it fussy baby syndrome: 780.91. See: http://www.Pregnancy.Org/article/curing-colic-4th-trimester-calming-reflex-amp-5-ss.

Heartburn: If the fourth trimester approach doesn't seem to help your frequently-crying baby, see your doctor. A very small minority (3%) of babies will have heartburn due to "acid reflux", and may respond to medical therapy. "milk allergy" is a frequent diagnosis, but true milk allergy is rare and often accompanied by blood in stools and other physical signs.

Many reasons: Without knowing a whole lot more about your situation, i can't guess why your baby is crying. Sometimes there is a genuine medical problem, sometimes it's more a matter of an anxious personality, and sometimes it's just blowing off steam. Visit your pediatrician to go over the situation in depth so you can get a good answer to your question.