Nothing really "causes" people to masturbate. It's a natural response to a physical urge. In fact, it has many useful purposes.
During adolescence it's a way of getting to know your body, physical sensations, how it responds, etc. During adulthood it's a safe way of satisfying sexual urges and fantasies and also releasing tension. It's perfectly healthy and normal. About 97% of all males admit to masturbating and about 80% of females. (Some say the rest are lying). Over 90% of adult males report mastrubating even when in a satisfying relationship with another person.
It has no negative health effects unless done too vigourously causing physical damage. In fact, it's thought that regular ejaculation either through sex or masturbation is good for your prostate.
It only causes problems when it becomes so habitual that it interferes with other parts of your life such as social interaction, responsibilities. As I always say, it's perfectly fine as long as you don't let it get out of hand ;)
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