Modified leaves and Cactus

From my high school biology class, I learned that thorns are modified leaves. Cacti are thorny plants Why are the leaves modified into thorns?

(1) For protection from animals that may feed or graze on them

(2) Probably to decrease the surface area of the leaves, to prevent excessive water loss that may lead to dessication.

Now, when you have plants sort of uprooted from their natural habitat, they act up and adapt to their new environment. This cactus still has succulent stem but look at their thorns, they're leafy now.