Since you don't want to make more details available as to why you need these answers, I will give you some helpful ideas. It is called premature ejaculation in the medical world. Though no exact cause is known for it, the causes can vary from person to person and from situation to situation. There can be both physical and psychological causes of premature ejaculation.
Below is a list of some major causes.
1. Conditioning
Several doctors are of the opinion that experiencing sex early can have a major influence on ones sexual tendencies and abilities later in life. For instance, a masturbating teenager that sets the condition for him to come quickly in order not to be caught masturbating can grow up having the problem of quick ejaculation.
2. Past Sexual Traumas
If one had a terrible sexual experience at a much early age, he can grow up with sexual anxieties that tend to last a lifetime. Such experiences can range from sexual abuse to being caught while masturbating.
3. Upbringing
Men who grew up with strict upbringing, where sexual activities were only believed to be right after marriage, may be unable to relax during sex, or may still hold on to that belief that sex is some kind of sin or wrong behavior.
4. Medical Causes
Some physical causes of this condition include multiple sclerosis, high blood pressure, diabetes, thyroid problems, being on recreational drugs, consumption of too much alcohol.
5. Psychological Causes
Some common psychological causes include stress, unresolved problems, depression, sexual anxiety bothering on ability to satisfy ones partner or following some sexual malfunctions in the past, emotional conflicts, etc.
I hope you found the content above a bit helpful for your question.