Knock first, don't enter

Pain shoots up my body
Into my eyes
That burst into tears
But not without making a stop
At my heart.
It wasn't polite enough to knock
But instead barged in.
I was unaware
Unguarded
And scared.
It came through to take you away
But you held your door tight
And you're still here.
I shouldn't be surprised
But I am
Because how could someone like you
Meet someone like me
Deal with someone like me
And yet still love someone like me?