What are the reasons for having pimples on the body? Especially at the back & face.

Many causes. Usually increased bacteria, sebaceous production, or hormones.
PIMPLE REASONS. Hormone changes are responsible. Certain foods aggravate acne. No nuts or peanut butter; no chocolate; cut down on dairy products. By the time you are 23 or 24, it should disappear by itself. If not, see a dermatologist who will probably prescribe an antibiotic.