He was a genius. He didn't need the instructors to tell him that, his so-called peers did it often enough. They were good to go pub-crawling with, but he had a feeling several would not have passed transporter theory without his help.
And now one of his "instructors" doubted his math, his ability. Well, Montgomery Scott wouldn't stand for that! This was a matter of pride, reputations were on the line. Mainly his, but that wasn't the important bit.
Of course you could send a grapefruit to an adjacent planet. That was ridiculously simple. What's more, Scotty was talented enough to send a life-form! Okay, he'd never done it before, but the equations were flawless and there was a logical progression here. People didn't like to push limits too quickly at once, but he knew how far he could go.
It was surprisingly easy to steal Admiral Archer's dog. The beagle loved people, and he particularly loved a bit of brandy, which Scotty had given him a nip at to make him more compliant. The dog obediently trotted up to the transporter pad, Scotty punched in the equations, and the dog was beamed away.
Only later did he realize the dog hadn't gone where it was supposed to. Oops?