A lot of things can cause bed-wetting, and identifying the cause can be an important step in resolving the problem.  Some of the causes of bed-wetting are:

Some children do not yet have bladders that are fully developed in size which makes it harder to hold the amount of urine produced over night.
The nerves that control the bladder can also be slow to develop, which means that for some children a full bladder may not be enough to wake them.
Some children may experience a deficit of anti-diuretic hormone (ADH), which helps to limit the amount of urine created, is otherwise. A deficit of ADH can cause too much urine to be produced, which can cause night time enuresis.
Anything that causes stress, like starting a new school or a divorce, can trigger bed-wetting.
Just like in adults, a urinary tract infection or other kind of infection can cause incontinence, which may manifest as bed-wetting.
Bed-wetting can be a sign of sleep apnea caused by inflamed or enlarged tonsils or adenoids. Other symptoms of sleep apnea include snoring, ear or sinus infections, sore throat and daytime drowsiness.
If a child is dry at night for a few months or more and then begins bed-wetting, this can be the first sign of diabetes. Other symptoms include passing large amounts of urine at once, increased thirst, fatigue and weight loss that does not result from a change in diet or exercise.
Constipation can put pressure on the bladder, which can decrease the bladder’s capacity and cause bed-wetting.
Bed-wetting might be a sign of some other problem in your child’s nervous system or urinary tract. This is rare, but bed-wetting might be the first sign of one of these more serious problems.