Frustration

This smile has a deception...
It's the smile of frustration...
It's hiding something.
That certain thing.

It's the smile
Of i like life
Yet i'd cut my neck with a knife.
The smile everyone has once in a while...
The smile of i'd kill you
Yet you are lucky i love everything
I love the worst
I love the best
And what's in the middle.
And to make a riddle...

You like it when it's bad.
You like it when you're sad.
You like it when it's good.
You like it when it gives you the happy mood.

Everyone has it.
Some say it gives you a hit...
Once in a while...
Yet again it makes you smile.

It's not grass...
It can't be put in a glass...
It's what you and I both have...
What am i talking about that can contain both sad and glad?