Why people get addicted to drugs?

Because they're either physically addicted, meaning their body has gotten so used to the drug it can't function properly without it.

This is normally caused by the fact the drug 'replaces' certain chemicals in the body, so the body stops producing them, so when you stop using the drug, you don't have that chemical. Which causes problems.

Ofcourse, slowly coming off the drug using lower and lower doses encourages your body to start producing more of the needed chemical again.

Or, they're addicted psychologically, which simply means they love the feeling so much they can't stop. OR that they've been using so long it's become a habit, like biting nails or playing with your hair.