Sounding smart

I read stuff all the time that I find plodding and fluffy. Sometimes I actually wake up long enough to realize "this writer is trying to sound smart." Big words, elegant variation, literary devices, fancy sentence constructions, long sentences, abstraction.

How come some of us naturally want to write that way?

Are we showing off? Are we insecure? Do we really think dense writing sounds smart? Do we fear that simple writing sounds stupid? Do we have the learned mindset that writing is a different thing from ordinary communication?