If you ever stopped to count the number of times the average American uses the word "like" in a given sentence, it's pretty eye-opening. A recent story in Business Insider found the term has four major functions in American English.
It's a filler word used mainly for pause and flow." It's an adverb that can be used as "a hedge, that's used for approximation." It's also used as "a discourse particle, to emphasize a point."
The use of the term is also a signal to to others that you are being informal and friendly. A big reason why it's overused by younger people.
So how did it become so popular? You never hear people dropping a ton of "likes" into their speech in movies from the '40s. The term gained popularity within beatnik culture of the '50s and then diffused into culture at large. The term was lampooned in the 1982 novelty hit "Valley Girl" by Frank Zappa.
This led to a popularization of the Valley Girl stereotype in movies and TV.
Now, you like hear the world "like" everywhere in the country.