- pain (e.g. colic, teething, sore throat, injury)
- discomfort (eg. a wet nappy, cold)
- if their carers are stressed or upset.
The first four are fairly obvious causes, but many parents forget about the last one. If the parents are arguing, overtired, upset or stressed, the baby will rapidly sense this and become upset itself.
The obvious ways in which to deal with a crying baby are to deal with the cause of the issue, but becoming upset about a crying child will only make the situation worse.