Saudade (Portuguese): The feeling of longing for someone that you love and is lost. Another linguist describes it as a "vague and constant desire for something that does not and probably cannot exist".
When Derek leaves Beacon Hills for a camping trip with his family when he's 14 he feels it for the first time—a deep sense of loss that settled in his gut for the entire trip. It wasn't a sad feeling, necessarily. It was the feeling that something that had been there before was no longer present. When his father asks him what is wrong, he just shakes his head. Uncle Peter gives him a knowing look, but Derek can't possibly understand what it is Peter knows.
Derek mentally shakes himself and then goes to his sister Laura and tugs her braid, already transforming himself into his half wolf form as he scampers off.
When he gets back to Beacon Hills, Derek feels freer, as if the thing he lost has suddenly been found, and he puts it out of his head.
He doesn't leave Beacon Hills against for another year. Although the Hale pack has always been fairly stable and well-respected in the supernatural community, it seems his father and mother and older siblings and all other adults in the pack are always off dealing with some problem or another. There doesn't seem to be enough time to take any more camping trips, so he and Laura and their younger siblings and cousins make due running in the preserve around their house.
The next time he leaves Beacon Hills, Derek hardly notices the feeling of loss come back again, he's so weighed down by the guilt and sorrow of losing his entire family to his stupid mistakes. If anything, he attributes the feeling of loss to the death of everyone in his family besides Laura.
So they move on and away, Laura trying to keep her and Derek together and Derek simply trying to escape the guilt he feels every time something reminds him of his family. They finally settle with a mated werewolf couple in New York who were friends of their parents and willing to help a brand new alpha and her brooding, quiet, little brother.
That's probably worse than anything for Derek, as the couple, Cate and Aaron, remind him daily of his Uncle Peter and Aunt Jessica. Derek still remembered hearing stories of how Peter saw Jessica as she sat down in class their freshman year of high school and they had been inseparable since. Peter used to joke that he felt like someone has died when they were away from each other for too long. Jessica had went away for college, but said that she felt so lost, so sad, when she was gone that she transferred closer as soon as she could.
It was almost unheard of, his father said, that Peter and Jessica's bond was that strong. His father said he could feel Derek's mother when she was near or in times of strong emotion, but to feel a loss from separation? He had never heard of it. Derek used to dream that he would find a bond that strong. He could almost picture the girl—blond hair, blue eyes, a smile as big as her face—but he stops imagining her after a blond hair, blue eyed girl, with a smile as big as her face and crueler than he'd ever seen, lit fire to his family. He stops dreaming he'll find any kind of bond. He knows he doesn't deserve one.
When Derek goes back to Beacon Hills 8 years later he is so focused on his sister's death and revenge and anger and guilt and every pent up emotion he's been repressing since his family's death that he doesn't even notice that the sense of loss eases up for the first time since the fire. Doesn't even notice when it goes away completely as he yells at two kids wandering the woods.
When Stiles is 7 years old, his mother dies. The first few days after she passes, he's in shock. He doesn't cry, doesn't shout, doesn't talk, really. He just stares, not at anyone in particular, but off into space. At the funeral, though, he finally snaps out of his shock. Unfortunately, it is into a panic attack. He's gasping for air that his lungs can't seem to find. He's shaking and falling, his legs unable or unwilling (he's not sure) to hold him up. He vaguely understands that there are people around him, shouting, crying, overwhelmed with the emotion of both a funeral and their own helplessness, but he can't find the energy or the focus to care.
But then there's a boy, a teenager, really, holding him close, rubbing his hand in circles on Stiles's back. Later he thinks it might have been the child of one of his mother's friends, but he never got a name and it never seemed to matter. All Stiles can think about is the boy holding him, ordering him in stern, but quiet tones to breath with him. He can feel the boy's lungs expand as he takes a breath, and Stiles tries, he really tries, to breathe in sync with him.
Finally, the attack seems to subside and Stiles notices the people around him. He sees his mother's sister, terrified and paralyzed, unable to help. He sees other funeral goers equally scared and helpless looking. But it's his father's face that gets to him. The look on the Sheriff's face is enough to harden Stiles's resolve to not show this weakness to his father again. He can't be a burden.
Stiles hears his father thank the boy, after a moment. "I… just, thank you. I don't know what you did, but you calmed him down. You made him okay."
"It's no problem. My older brother gets panic attacks like these, since he got back from Afghanistan. PTSD, I think they call it. He said with the way our family heals, he had a duty to fight, but I don't think he counted for emotional injury. Sometimes you just have to make him breathe."
But then the boy blushes, even the tips of his ears going red, as if he's said too much. He scampers back to a large group of people that look remarkably like him. His family, Stiles guesses.
Two weeks later, Stiles wakes up, gasping. He has a feeling in his stomach, one of deep loss. At first he thinks it's something different than the loss of his mother, but he shakes his head. What else could it be?
For three days, Stiles is walking on edge—feeling just one sad or guilty thought away from a panic attack. A few days after the funeral, his father had him see a counselor, to help with his grief. She had given Stiles some short-term coping techniques and some medicine that was supposed to help, but for a seven year old who lost his mother, she said only more therapy would truly help. The coping techniques help, to an extent, but he stays away from his father as much as possible that weekend, trying not to let the Sheriff see that anything is wrong with his son.
After three days, the sense of loss lightens a little. Stiles isn't sure why, but he doesn't question it. He just sighs, relieved that a panic attack isn't so close to the surface.
A year later, the feeling of loss returns. Stiles attributes it to it being so near the anniversary of his mother's death, but it doesn't go away. He learns to live with it. He's long since learned to control or hide his panic attacks from his father and his best friend Scott, and he figures this is just one more thing to add. His father is so wrapped up in a huge arson case that Stiles barely has to hide it from him at all.
Scott's mom catches the tail end of a panic attack, as Stiles is hiding in the bathroom. He begs her not to tell his father, and though she gives him a side-eyed glance, she complies.
Stiles knows that he's too old to believe in fairy tales. Girls could still talk about them, but at 8 years old, boys should only care about action figures and superheroes. He can't help believe in soul mates, though. He knows that his mother was the end all, be all for his father. He can't let his father know he is having panic attacks. He has to be strong for his father, because his father had his world ripped away from him and he was still being strong for his son.
So Stiles hides and controls and ignores the feeling of loss that settles in and he stays strong for his father.
8 years later, Stiles is so caught up in Scott's transformation in a werewolf that the adrenaline and fear and sheer novelty of the situation keeps him from noticing the sense of loss lessening. When Derek Hale is screaming at him and Scott for trespassing, Stiles is far too afraid and nervous to notice that his sense of loss is gone completely.