Intensive describes things that are extreme, highly dramatic, or just plain thorough. An intensive chemistry course meets six days a week for five hours a day. If you're in intensive care, you're getting close medical scrutiny, 24/7.
Intensive is intense, which comes from the Latin for “high strung,” plus –ive, meaning “having the qualities of.” It’s an adjective that cranks up whatever it modifies. Doing too many intensive activities can make you high strung indeed, like taking that intensive chemistry class or participating in an intensive study on the effects of sleeplessness by staying awake for four days straight. That might get you put in intensive care.