Disclaimer: Torchwood is not mine.
Tosh had always been the practical one, the sensible one, growing up. Trapped between Sadayo, the eldest and a wild child, and little Chiyo, naive and overly imaginative, she'd had to be. So if she was so practical, how she possibly have such bad judgment when it came to love?
First there was Owen. Her bitingly sarcastic co-worker. There was absolutely no reason she should be attracted to him. He was rude, immature, and honestly, not the most attractive man she'd ever seen. But sometimes he smiled - not smirked, really smiled - and she just fell apart. She thought he at least liked her, a little. But she would never have the courage to speak up. So she watched him when she hoped no one was looking, praying that no one would guess.
Then had come Mary, the blonde who had dropped so abruptly into her life one night when she'd been depressed. Gwen and Owen had played a joke on her, and though she shouldn't have let it get to her - "You know, Tosh, that stick up your arse has a stick up its arse." So she was at a bar, drinking, and Mary had sauntered over. A stranger who knew all about her, who gave her a pendant, a curse.
That damned pendant. She'd found out about Gwen and Owen's affair because of it. The new girl had replaced Suzie, and she now had the man Tosh wanted. But she didn't love him, Tosh was sure of that. Gwen may have loved her boyfriend, but her eyes followed someone at work, and it was never Owen, or never just him, anyway. But Owen... "Catholic but grateful, I'd bet." He pitied her. They both did. Ianto was too wrapped up in his own pain to pity anyone, and Jack was unreadable. But the pity... God, she couldn't stand the pity. She thought they cared about her, that they saw her as a friend, the way she saw them. But she'd been wrong, apparently.
So when Mary had popped up again, Tosh had thrown out her inhibitions. She'd kissed Mary, she'd slept with her, only to find out she was an alien. An alien who'd killed more people than she wanted to know, had ripped out their hearts and eaten them. Mary had held a knife to her throat, and yet, Tosh still loved her. Because what Mary said was true; she'd never pitied Tosh, no matter what. And Owen, who would rather see her die than Gwen, and who'd tried to deny it, so obviously did.
She was left with that. An alien who was dead, burned up in the sun, who had been homicidal and yet maybe had actually loved her. And him. The man she still saw every day, who was completely blind to how she felt about him. It wasn't practical to still love him even after she knew there was no hope. She kept it hidden, though, because she had no choice. Because Toshiko Sato was always practical, and it was practical to stop your heart from shattering - even if it's already snapped in two.