4 letter's 1 word: The Secret behind those words

I used to look into the sky
Teardrops through my eyes
You're the reason why I lived
But now you're the reason why I'll leave.

You used to make me laugh.
You used to make my eyes drop
You used to make me smile,
But now you only make me cry.

You're the one who makes me feel the pain,
You're the one who makes me feel this madness through my veins.
Somehow I feel my heart aching.
Somehow I feel my heart breaking.

I used to loved you, but now I hate YOU.
This is my poem for you,
This is my hatred towards you,
4 letters 1 word, the secrets behind my words.