There are hundreds of causes (including alcohol, chocolate, tea and coffee) but it is often useful to consider what has changed prior to the onset of the itch. 

The main causes start with the letter "S":

 

Soft Stool

Seepage from Sphincter and Skin tag Surgery

Sweat leads to fungal infections (candida)

Sexually transmitted diseases

Sensitising agents (soap etc)

Skin diseases (sclerosis psoriasis)

Steroids

Systemic illnesses (diabetes, liver/renal failure)

 

If no cause is found then most patients do improve by Stopping Scratching.

If symptoms persist then a biopsy to exclude worrying causes must be considered [ppAIN.ppt]