Signals
Crying is the way that babies signal they need something. When you're trying to find out why your baby is crying, consider the following possibilities, which can be tried in any order.
Hunger
Many babies don't feel hunger at regular intervals, so there may be days when your baby wants food frequently and other days when he wants it less often.
A baby who hasn't had enough to eat will usually cry at the end of a feed as well as before the next one.
If you're not sure whether your baby is hungry, offer a feed and see what happens. If he takes a little milk then refuses more, hunger is unlikely to be the main cause for his crying.
Sucking
Sucking is comforting to babies, so there may be times when he doesn't want the bottle or a full breast, but he does want to suck. If you think this might be the case, offer him a dummy or let him suck on the breast he most recently fed from.
Needing to be held
If a baby is crying because he wants to be held, picking him up and cuddling him will quickly stop the tears.
Carrying or gently rocking your baby is probably better than vigorously joggling him up and down, because this may prevent him from drifting off to sleep.
Don't worry that you'll 'spoil' your baby by picking him up whenever he demands to be held – you won't. A backpack or a front-carrying sling may be useful, and some babies like to be snugly wrapped in a blanket while being held.
Boredom
A baby won't sleep if he isn't tired, and in this situation he may well cry from boredom.
Try playing with him or sitting him in a baby seat in a room where there's a lot going on (for example, the kitchen while you're cooking). A new mobile above the cot, or a new ring or rattle to touch and hold, may help.
Tiredness
Too much excitement and stimulation can sometimes overtire a baby, making him irritable and unable to sleep.
Try putting your baby down in a quiet, darkened room and leave him for a few minutes, even if he's crying, to see if he will settle and go off to sleep.
Thirst
Babies may cry because of thirst, even when they're not hungry, especially in hot weather or a very warm room. Try offering a little cool, boiled water in a bottle or from a spoon.
Wet or dirty nappies
These bother some babies more than others. But even if your baby doesn't seem upset by them, it's worth changing the nappy as soon as you realise it's wet or dirty. This helps to prevent nappy rash, which would almost certainly cause your baby discomfo.
http://www.netdoctor.co.uk/health_advice/facts/whybabiescry.htm