Tris, as a rule, is not a very physically affection person.
You think about this from time to time, considering why, you could chalk it up to her childhood, her family's lack of affection, but that doesn't seem to fit because her other family is affectionate enough, more, even, then most families, and hasn't she been with them longer than she was with her blood kin?
Then you consider her power, she's not secretiuve about her days of losing her temper near constantly, and the sparks she used to grow automatically and usually unintentionally sparking anyone who get too close but that couldn't be it either because she could control that now, had been able to for a long time, and you don't think you make her that angry most of the time.
But one day it clicks for you, and you nearly want - no, not want, need to - laugh. They, the four of them, don't know you're watching, and they're very close together, smiling and laughing - depending on who they are - over something, and there's not a one of them who's not touching another, arms looped around each other absentmindedly.
She's theirs, you think. They got to soon enough that she could trust them, she could love them, but she'll never be able to get that close to anyone else. It's possible she never even wants to.
You turn to exit, not angry, but disappointed a bit. You can't be angry, you don't even think she's aware of that limitation, and she certainly isn't to blame. At least you know they appreciate how lucky they are, most of the time. At least you know she'll never lose them, and they'll never lose her.
She'd call you crazy if you told her you thought the rest of the world lost out, but they'd agree.