Disclaimer: I do not own anything in the Harry Potter universe.
When Harry began to study how to become an animagus, he had not expected his form to be a small, sleek black cat with large green eyes. Neither had Ron, although once Harry had transformed the first time, he said it fit.
"You can be sort of aloof when you're troubled. And don't tell Hermione, but I've seen Crookshanks be as loving as any dog, and you are loyal to your friends. Maybe that's all the transformation was looking for," Ron said.
Either way, once Harry became an animagus, it opened a whole new world to him. As a cat, Harry could wander around the whole school whenever he wished, and there was less risk of being caught than with his invisibility cloak. Harry knew he should immediately tell Hermione that their hard work had paid off, but first he wanted to just enjoy being a cat and meander for a while. After a couple of hours, though, he made his way back to Gryffindor Tower.
What Harry didn't count on was Ginny catching him in the Gryffindor Common Room on his way to Hermione. Ginny asked around, but while the girls cooed over him and Ron couldn't stop smirking, no one laid claim to Harry. Finally, she decided to keep him.
"I'll call him Barnabas," she said, stroking his head.
Harry rather enjoyed the stroking, and only barely noticed the unfortunate name choice.
There was a heated argument between Hermione and Ginny concerning the school rules about only one pet per student. Finally, Hermione convinced Ginny to take Harry, or 'Barnabas', to Professor McGonagall to ask about an exception.
Professor McGonagall let Ginny and Hermione into her office, and listened as they explained the situation. The professor took Harry from Ginny to examine him (apparently he was fairly young, perhaps a little underweight, and definitely male), and kept holding him as she discussed school rules and finding to whom 'Barnabas' originally belonged.
Harry couldn't resist exploring this new, softer side to the strict professor, and allowed himself to relax in McGonagall's lap. Oddly, McGonagall's lap reminded him of a soft, warm sensation from a long time ago. Purring came instinctually, just like flying came instinctually his first time on a broomstick. Ginny made a small exclamation of delight as Harry purred. Seeing how much his friends loved it when he simply allowed himself to act like a cat, Harry gave in to his instincts even further and began licking his professor's hands and sucking on her fingers.
For a bit this had the desired result. Hermione almost squealed, a much more girly sound than he'd ever heard from her before.
"Professor, why is he doing that?" Hermione asked.
"Poor lad, he was taken away from his mother too early. Cats do that..."
Harry froze. All of the warm, fuzzy cat instincts dropped away, leaving only cold, human reality. His chest suddenly too tight, he leapt from his professor's lap and ran as fast as he could. Hermione called "Barnabas!", but he was already several hallways away.
As soon as Harry made it to one of the secret passageways, he transformed back into a human and slid down the wall. For just a few moments he'd felt safe within a mother's love. Harry drew his knees to his chest and rested his head on his arms in misery.
Sometimes, even the most social cat just needs to be alone.