"Sneaking in without cause,
pulling on innocent heart strings.
Bringing smiles and laughter,
secretly inducing hypnotic lies."

^^I love this opening verse, I found it be beautifully written, the sadness is overwhelming while the depth and emotion pull me right into the piece.

"Stories from this person,
are they true or false?
Learn the ways of the monster,
a depression deep inside."

^^I found this stanza to hold alot of power, but don't think you need the "a" on the last line, it works just as well without.

"Tearing you down slowly,
leaving you weak and fatigued,
weary from attempting to fight.
The sadness persists another day."

^^You manage to portray how depression can affect a person very aptly here, the feeling of hopelessness and despair, however I don't think you need "the" in the last line.

"Finally you're down completely,
the monster's goal satisfied.
A friendly foe that enters,
bringing down your spirit. "

^^What a beautiful closing..filled with so much lenacholy yes, but nevertheless beautifully and elegantly written.