If you rush to the carwash every time a speck of dust lands on your fender, your friends probably describe you as obsessive about keeping your car clean. In other words, you care way too much about your car's cleanliness.
Whenever someone is wild about something to an alarming or unnatural extent, you can say they're obsessive. If you go berserk when your neighbor’s soccer ball lands in your flowerbed, your neighbors might think you’re obsessive about your garden. If there’s a person you think about every moment, night and day, you may be obsessive, or maybe you’re just in love.