There are many possible causes of anal itching, including:

Fecal matter adhering to the skin around the anus (in cases of diarrhea or loose stools)
Hemorrhoids
Certain foods or drinks that can irritate the anus; coffee, tea, beer, cola, and chocolate
Citrus fruits such as tomatoes, oranges, or grapefruit
Diseases that can affect the anus; anal abscess, fissure, and fistula
Cancers (anal canal, colon, and rectum)
Skin diseases; psoriasis
Dermatitis (creams, detergents, etc., that cause skin irritation and allergies)
Infections (bacterial or fungal)