It can be a BDSM thing — collaring is a pretty common way a Dom demonstrates... well, it can demonstrate a few things. If it's scene specific, then it's usually about the specific kink of submission — but in long-term D/s relationships it's not unusual for the sub to want, request or be given a collar almost as a sign of subtle — but public — ownership.

（ he follows sasuke to the bed, where the tray of food is set out there, and sprawls down beside him. the mattress is really too fucking small to make anything comfortable, but he braces long legs against the wall and shimmies downward until he can put his head in sasuke's lap. ）

It's hot, but it's not like it's a necessary component for me.

（ he reaches up, and drags his thumb against the column of sasuke's throat, feeling the flex of a swallow beneath his touch. it's maybe the first time he hasn't prefaced a statement of this nature with a caveat, or a disclaimer that sasuke doesn't have to do it, or feign interest for his own sake — if the boy is attentive, he'll take it as the sign of trust it is. ）

You'd look gorgeous wearing one, though. If you ever felt like trying it out for a scene.