Scratching has traditionally been regarded as a way to relieve oneself by reducing the annoying itch sensation. However, there are hedonic aspects to scratching, as one would find noxious scratching highly pleasurable. This can be problematic with chronic itch patients, such as ones with atopic dermatitis, who may scratch affected spots until they no longer produce a pleasant or painful sensation, instead of when the itch sensation disappears. It has been hypothesized that motivational aspects of scratching include the frontal brain areas of reward and decision making. These aspects might therefore contribute to the compulsive nature of itch and scratching. Contagious itch
Events of "contagious itch" are very common occurrences. Even a discussion on the topic of itch can give one the desire to scratch. Itch is likely to be more than a localized phenomenon in the place one scratches. Results from a study showed that itching and scratching were induced purely by visual stimuli in a public lecture on itching. The sensation of pain can also be induced in a similar fashion, often by listening to a description of an injury, or viewing an injury itself. There is little detailed data on central activation for contagious itching, but it is hypothesized that a human mirror neuron system exists in which one imitates certain motor actions when they view others performing the same action. A similar hypothesis has been used to explain the cause of contagious yawning.
Xerosis: dry skin, frequently seen in the winter and also associated with older age, frequent bathing in hot showers or baths, and high-temperature and low-humidity environments. Other medical disorders
Diabetes mellitus, a group of metabolic diseases in which a person has high blood sugar
Hyperparathyroidism, overactivity of the parathyroid glands resulting in excess production of parathyroid hormone (PTH)
Iron deficiency anemia, a common anemia (low red blood cell or hemoglobin levels)
Cholestasis, where bile acids leaking into the serum activate peripheral opioid receptors, resulting in the characteristic generalized, severe itching
Malignancy or internal cancer, such as lymphoma or Hodgkins disease
Polycythemia, which can cause generalized itching due to increased histamines
Psychiatric disease ("psychogenic itch", as may be seen in delusional parasitosis)
Thyroid illness
Uraemia – the itching sensation this causes is known as uremic pruritus

Medication
Drugs (such as opioids) that activate histamine (H1) receptors or trigger histamine release
Chloroquine, a drug used in the treatment and prevention of malaria
Bile acid congeners such as obeticholic acid

Related to pregnancy
Gestational pemphigoid, a dermatosis of pregnancy
Intrahepatic cholestasis of pregnancy, a medical condition in which cholestasis occurs
Pruritic urticarial papules and plaques of pregnancy (PUPPP), a chronic hives-like rash

Other
Menopause, or changes in hormonal balances associated with aging
Terminal illness

Mechanism
Itch can originate in the peripheral nervous system (dermal or neuropathic) or in the central nervous system (neuropathic, neurogenic, or psychogenic). Dermal/pruritoceptive
Itch originating in the skin is known as pruritoceptive, and can be induced by a variety of stimuli, including mechanical, chemical, thermal, and electrical stimulation.