The knock knock

Every time you hear the knock you know a reward is incoming.
Imagine the conditioning of this.

Knock
Knock
?Cum?

You reorient towards the next cock.
You start sucking
Working while waiting.

Knock
Knock
?Cum?

In the future when it just knocks randomly on your door. You will expect a cum reward. Instantly you will be horny for serving cock.
The person knocking most likely don't know, but you will feel disappointed when answering the door.

Knock
Knock
Nothing