Tom Stafford determines its cause:
Irregular rewards have a special power to enforce repeat behaviour, something discovered by psychologists in the early twentieth century, and known for centuries by people who organise gambling (would anyone play slot machines if they just predictably gave you back 80% of the money you put in each time?).
Email drips into your consciousness during the day. Each time you check it you don’t know if you’ll be getting another boring work email, which isn’t very rewarding, or some exciting news or an opportunity, which is very rewarding. The schedule of these constant opportunities for surprise hooks us into checking email. To avoid it, you just need to fix your email so that you collect it all at once at regular intervals, such as every hour or twice a day, rather than checking each email as it arrives.
The Dish 