When my dog was young but physically mature, he was ready. I figured it would be a good step in the progression to suck him off, preparing for the time when he would tie me. I got naked, put the blanket on the floor, got him excited, stroked his knot and got him humping. Soon enough he thrust his knot out of his sheath into my hand, and I clasped it, continuing to swell and throb, filling my hand. I was trembling with anticipation and a little fear that somehow something might go wrong. Then in a surprisingly atavistic state of mind I leaned in and sucked his shaft into my mouth. I didn't overthink it! I just wanted it, to suck it dry. But he was fidgeting, dancing around with his legs. I took a moment to pull his cock back between his legs in "tie and turn" configuration, I resumed sucking with his tail in my face, and he settled in with a steady pull and his tailhole pulsing. Oh yeah I sucked him dry. I swallowed every drop. When he was done I had an explosive ejaculation of my own for him to gobble up.
So yeah, the main thing that went through my mind in that moment was the animalistic urge to do it! I'm not "subby," I'm not into humiliation, but I do take a lot of pleasure in pleasuring my boy. The firm, warm fleshiness of his shaft in my mouth was strangely compelling, and we did it countless more times.