Hermione Granger, being the child of two parents with a surgical practice, grew up in a pristinely clean home. Not only had the Grangers not had any other children, they'd never permitted their only child to have a pet of any kind. Not even so much as a goldfish.
Being so obsessed with academic performance as such a young child, coupled with her strange "abilities", had served to completely alienate young Hermione from her peers. As a result, she hadn't had any friends, and thus none with pets, and so consoled herself with books.
Thus, when Crookshanks had leapt, practically into her arms, at the store, she'd fallen completely in love with him. He, apparently, returned the favour. When Professor Snape said horrible things to her that made her cry, he would crawl up next to her and lick her tears. When she had a row with Ron or Harry, Crookshanks would meet her at her dormitory door, and flop down in front of her, half on his side, half on his back, and beg to have his tummy rubbed. She'd taken to calling him the "toll cat," from an old Muggle children's story about goats crossing a bridge with a troll underneath it. And when she was frustrated beyond measure her second year, taking on far too many classes, she'd collapse into her bed, wanting to scream with frustration, and Crookshanks would head-butt her, looking for love.
She'd read all about them, of course, cats, and knew that the funny rumbling noise they made in their chest was a sign of contentment, and, whenever he was misbehaving and she'd make to scold him, he'd leap into her arms and began purring loudly. Whenever she was upset, or not feeling well, or had cramps, he seemed to know, and he always lavished attention on her. The very night she rescued him from that horrid store, he'd jumped up onto her bed and began paddy-pawing her, purring loudly and cooing, which she knew was a sign of delirious happiness, and marking her by rubbing the side of his nose on the bridge of hers, until he finally nuzzled into her neck, stuck his nose right in her ear, and purred himself to sleep, one paw draped possessively across her neck.
He frequently would leap up onto the bed with her, and paddy-paw her bushy brown hair into an impenetrable nest of tangles, but she couldn't bring herself to be frustrated with him since she knew he only did it out of affection and contentment.
She slept like this with her familiar every night since she'd gotten him, and had discovered that his nuzzling and purring were quite relaxing. Many times she'd laid in bed to study, because the half-Kneazle would invariably leap at the chance to massage her neck, which was almost always sore from being bent at an unnatural angle while she studied at a desk.
She wondered how she'd ever lived without him.