There are so many reasons that a baby cries. A baby cries because they can not talk. It is their way of expressing their needs and emotions.
One of the cool things that I have picked up on are Ace’s different cries. He cries a certain way when he is hungry. Another way when he is tired. There are also a lot of physical signs that help diagnose the reason for them crying such as rubbing their eyes, yawning, turning their head, arching their back, etc.
Take a look at a post from Mom Exchange called How To Calm Your Crying Baby. The author does a really good job expaining all of the little signs to look for when trying to figure out why your baby is crying.
Try to pay attention and remember what your baby was doing before they started crying. Babies usually gives hints and clues that something is wrong before crying. If you know these warning signs that your baby gives off you can prevent the crying by fixing what is wrong before it gets too bad.