What makes "like" and "so" popular?

The practice of doing so is actually a field of research and the use of these words in such a manner can be classified as fillers, used while someone is busy grasping what they want to say and so on.

From Wikipedia we get a general overview of this:

Fillers are parts of speech which are not generally recognized as purposeful or containing formal meaning, usually expressed as pauses such as uh, like and er, but also extending to repairs ("He was wearing a black—uh, I mean a blue, a blue shirt"), and articulation problems such as stuttering. Use is normally frowned upon in mass media such as news reports or films, but they occur regularly in everyday conversation, sometimes representing upwards of 20% of "words" in conversation.[citation needed] Fillers can also be used as a pause for thought ("I arrived at, um—3 o'clock").

The idea of whether these are valid in language or not depends on whether they follow the rules of the language, and hence, can't be answered with a simple yes or no in this case. What we can say is that people do stand by the rules of syntax of their language and so can conclude that, for the most part, such 'fillers' are 'correct'.

On the other hand, there are certain individuals and industries that hold different levels of contempt towards users, and this can be appreciated; if, for instance, you were listening to the radio and the DJ incessantly hummed and hawed with (what is to the listener) superfluous speech, then the satisfaction of tuning in would be degraded.

If we examine that last part in a little detail it is easy to see the variable conclusions one could draw, either of a person or a company, based not on what is being said but rather what is in between what is being said, the gaps et cetera. I think it's safe to say that a great affinity towards this is not held by many.

To think about how it became so 'popular', I would reduce that to 'common', using the meaning of such which categorises social status. This might be a bit of a bold step, but I believe that where the above issues constrain one from committing, the lack of those constraints allows for another to become complacent and further habitually commit the act. That's a sweeping remark, I know, only skimming the surface of an idea; and there is without doubt a whole lot to delve into in order to give this thought any credibility; I, for one, think it would be an interesting line of research - right now I don't have the time or the inclination to do so, but encourage you to pursue should you care.

• why? Because it gives time for the speaker to figure out the rest of what they're going to say. It sounds better than 'um' (though not much).

• is it ok? It is contraindicated in any written language and in any spoken language more formal than talking to friends.

• is it grammatical? Yes in the sense that it has governing rules...come to think of it the rule seems to be that it can go anywhere.

Well, I have a partial answer for

So

So is a transition word. So, using it is grammatically correct. Though, the overuse can be annoying.

• Like is a function word and it can be:

  1. a conjunction "I felt like I'd been kicked by a camel";
  2. a noun "A church interior the like of which he had never seen before.";
  3. an adverb "there was this funny smell—sort of dusty like.";
  4. a preposition "They were like brothers";
  5. an adjective "I responded in like manner".

• So is a fucntion word as well and it can be:

  1. an adverb "I was so glad to meet him.";
  2. a conjunction "I worked hard so that I could buy a car."

Such category of words are much more used in discourses since you need them to form something that has meaning and is coherent. They are like the glue that allow you to write/speak and do it easier.

I think this is the main reason of why they are so popular. They serve much more purposes than a normal noun would, such as car, for example.

Note: Examples for like have been taken from the NOAD.