Ans: Because in reactions, metals always lose their electrons being very electropositive (bigger in size in their period and having less number of electrons in their outer most shells) and non-metals gain these electrons being very electronegative. So metals cause reduction of other elements/compounds and they themselves oxidize and while non-metals cause oxidation of other elements/compounds and they themselves reduce. That’s why they are known as reducing and oxidizing agents.
Hello, my name is Jack Sparrow. I'm a 50 year old self-employed Pirate from the Caribbean. 
