There's only so much a heart could take, even for an immortal.
The darkness never helped him sleep; instead it served as a cruel reminder to his unknown fate. His cell always felt cold despite Asgard's usual warmth and his long-sleeved tunic. He supposed it was because of his heritage. All because he was a frost giant.
He hated those words so much. Frost giant. If he could, he'd get rid of his blue skin. If he still had his magic, he'd claw it all off until it was nothing more but fragments of a lost mask. But deep down, he knew that wouldn't solve anything. He'd have to erase the frost from his DNA, pump it out his veins, slip it all right out before he'd feel better. But he couldn't.
And somehow he got it into his mind that his "brother" knew this. He thought the blonde knew Loki couldn't-wouldn't be able to change. This, in Loki's mind, was why the Asgardian prince never visited him. It hurt, it truly did, but he rationalized that he deserved it.
So Loki sat and endured his punishment of a century of solitude with the exception of guards coming to feed him. He sat in a fit of self pity, of self hatred, and refused to cry himself to sleep. His hair grew longer, but he didn't care.
It was painful for him, and he never spoke, not even to himself. He was expecting the darkness that had become a veil of his soul to gobble him right up.
Instead though, it brought Thor.