Toothless watched his human leave. He hated to be parted from him, but he sensed an importance... and more than just that in the boy's voice. He needed to do this. But Toothless would stay close by. The mention of this woman unnerved him. He remembered all too well what had happened to his boy the last time he grew affectionate for a woman. He remembered the instant attraction, that hadn't been too much of a problem... Besides the fact it was forbidden. He remembered the effort Hiccup had gone through to make her smile, he recalled all too well long flights and walks with her. He'd been rather fond of the girl himself. She'd been gentle with him, scratched him just where he liked it, and she herself had bonded well with the dragons. She had a fiery attitude and plenty of spirit. She'd challenged his human, but mostly in good ways.
He could still remember the morning the two had come out to greet him. Hiccup's scent was all over the girl and hers all over his. It had seemed that the courting dance was over and that the two had mated. Toothless had been quite happy with this, Hiccup had been very happy as had she, and oddly it made them all the more affectionate towards him. For a short while, everything had been good and right. Though their affection and attraction for each other generally had to remain very secret and hidden. It had all been very pleasant... But suspicions ran high in the higher classes of Berk. Hiccup's attention was never on the other women of class whose hands were offered to him. He never looked at them, Toothless saw no issue with this, but apparently it was wrong. Hiccup was forbidden to love her the way he did. The more suspicious Hiccup's father had become, the more suspicious all the court had become... The more it had taken its toll... Toothless knew Hiccup had not been willing to let anything stop him spending his life with her. He'd been looking for anything, any way to remain with her... She however, had become more and more depressed by it all, she'd often sought comfort in Toothless' wings. He'd spent many an hour beside her whilst Hiccup was doing his Princely duties. She'd cried on occasion. Toothless had hated seeing her that way. She'd been very frightened in the final few weeks they'd been together. Hiccup's father was becoming increasingly aware of her relationship with Hiccup and he was not best pleased. Toothless had heard rumour that he'd signed an order to have executed. But it couldn't have been true. For Stoick was not that cruel. And for perhaps another two weeks... Everything was ok again.
Then came the morning of training, the early session. Toothless and Hiccup had gone out before the sun had even risen to train and returned just after it rose. Hiccup had been going back to his chambers to see her, spend another hour alone in her company before breakfast in the Hall... Toothless had been going to catch up on some sleep with the other dragons. That plan had gone out of the window only minutes later. Hiccup had walked into the stables, his face pale, and his eyes red. Toothless had known something was terribly wrong in a heartbeat. Hiccup had replaced his saddle and insisted they go out again immediately. They spent hours out as Hiccup searched the village and surrounding forests endlessly for her. But no sign. They'd missed breakfast, and the lunchtime gathering... And Hiccup had missed several important appointments. Such a telling off Toothless had never seen.
Stoick had been raging at Hiccup for hours... But Hiccup had not flinched once, in fact, Hiccup had been far more snappy and confident. He'd yelled at his father which was something Hiccup rarely ever done. He'd been in much trouble after all that. Toothless had not been permitted to fly with Hiccup for days following that. Hiccup had been so miserable after she left. For months he was unreachable, emotionally he shut himself off. No matter what Stoick tried, Hiccup would not look at any other woman. Hiccup hardly paid attention to anything. He'd always been good to both dragons and his people, but with her gone... It was as though she'd taken a piece of Hiccup with her when she left, a piece that contained Hiccup's happiness and enthusiasm. And even years later... Something still seemed to be missing from Hiccup. Toothless just hoped someday, Hiccup would find the part he was missing. But he was fearful always whenever a woman was mentioned in regard to Hiccup, especially when she was strong and beautiful... For that seemed to be Hiccup's weakness. Toothless would watch him closely. They had left because Hiccup had tired of his father's lectures and trying to force him to marry Lady Heather of the Southern Kingdom. Hiccup would be a great King for Berk someday, but he was fleeing from his duties because of the way they were being forced upon him.
Toothless glanced out from his hide-away and watched as the small figure of Hiccup hurry away. He watched until he could no longer see the boy and made a solemn promise to himself, that no matter what it cost, he would find a way to make the boy smile again, a true smile that would never leave. He then looked out over the horizon, overlooking the kingdoms... Even if it meant tracking her down. He'd not rest until he found her, if that would make Hiccup truly happy once more. Toothless slipped out and headed for Berk, to ask for help from the one who would know best what she would do...

Toothless perspective. I wanted to establish the way Toothless sees things, how he feels about it all, and just what he's willing to do for his friend. Hoping it came out alright.