An "outsider's" = non-native speaker's opinion: "like" is mostly used in American English and is a parasite word. As such it catches easily, especially when one is exposed to the environment where others use the word a lot. As explained already, it's a filler like "er", "um", "well", etc.
Like I don't know!
Because it's like so awesome to be like soooo radical
Hello like how's it going like?
No... I have other habits
Oh I'm just sure your not :)
And decreasing rapidly!
Lot's of people do it to fill pauses between there thoughts or don't know other words to use to describe what they are trying to say.