The lines I can't explain

Ungrateful

"You're the best" that's what I was told,
Unless I became a twelve-year old
They welcomed me to my teens,
By blowing away all respect to smithereens.

I am not the child they wanted to raise,
That's what I was told - let alone any praise.
Respect can't be forced, it has to be earned -
For them, it is yet to be learned.

By the time came summer the fifteenth,
I had already heard the above for the umpteenth.
New sentences came in flying,
"I am a bad investment" - the astrologer can't be lying.

They come in hounding for the credits if any,
For any good I do - studies being one of the many.
"You still don't respect us enough" - they say,
It's my duty to respect them enough - come what may.

I am a failure as a son - others are better, I'm no good,
Get to hear it everytime - even while having food.
I revolt - "Just you wait for a few years more..."
They throw me to disinheriterance's door.

I am a disaster to them - a taint, a black sheep,
They say, ignorant to how much it cuts deep.
I am a bastard who shirks from duties, disgraced,
Still, "It's not how I was raised."

So sorry to you, but our situation cannot be undone,
Regards, your Ungrateful Son.