word usage - What makes "like" and "so" popular?

So, I was like, why does everyone say like and so in every sentence? Where did this trend come from, like, what started it, and is it actually grammatically correct to like, insert like into our speech in just about any position in a sentence?

Reward for anyone who can tell me the cause of its origin (i.e. cartoon, or star, or artist who uses these terms frequently and popularised its usage in this manner). And prove that overuse of any word is grammatically incorrect, or prove that there is no such rule for overuse.

Answer

Using "like" as in "this is, like, uncool" used to be strongly associated with Valspeak:

Many phrases and elements of Valspeak are stable elements of the California English dialect lexicon, and in some cases wider American English (such as the widespread use of "like" as a hedge).

This use of "like" is again mentioned in the Wikipedia entry on California English:

The use of the word like for numerous grammatical functions or as conversational "filler" has also remained popular in California English and is now found in many other varieties of English.

For a good description of "filler" usage see Fumble Finger's answer. A "hedge" is a word or phrase used to diminish a statement:

There might just be a few insignificant problems we need to address.

The party was somewhat spoiled by the return of the parents.

The idea here is that removing the bolded words would make the sentence stronger or more assertive. Using "like" as a hedge is extremely common and it seems to work as a universal hedge and may be used simply because it can be chained so easily with other fillers and hedges:

You, like, kind of smell.

I am, like, so hot!

This is, like, boring.