I'm lumping a few of my answers together in my explanation since the reasons are so similar.
I separate my top interests into 3 categories: Family, Friends, and the Unknown.
1: Protagonist V Brothers AND Protagonist V Dad
Interpersonal relationships fascinate me, and you can't get anymore personal and close than family members.
Bondage between brothers can stem from the usual sibling rivalry. Both boys argue about a game they are playing, and one decides to take revenge on the other. An older brother is throwing a party, but he doesn't want his younger brother hanging around and embarrassing him in front of his friends. The younger brother is being babysat by his older brother, and he tricks him into a hogtie so he can do what he wants without being bossed around.
Bondage between father and son is especially interesting since it's not typically nefarious when it happen. But I'm not one for simple games
I'm more interested in the reason why the dad would do something many would consider unacceptable to do to their child outside a fun game. Like, the son finds proof that his dad is cheating, and the father becomes desperate to keep his son from talking, so he ties him up until he can find a way to settle the mess he made. Or perhaps the son is so unruly, and he pushes things too far; the father comes to a regrettable decision in order to protect his son from hurting himself or others.
2: Protagonist V Rivals, Protagonist V Friends, AND Protagonist V Roommates
Bondage between rivals is simple enough since it provides both characters an immediate motivation to tie one another up. They're both in competition with one another, and in a desperate move to come out on top, one decides to eliminate the competition by tying up his rival. Competition could be anything: They both love the same person, and one ties the other so he can go on his date unimpeded. They're both vying for the same spot on a sports team or talent show or acting job, but there's only one spot open. Naturally, the conclusion would be to, again, limit the competition.
I speak of friends and roommates almost simultaneously although I prefer roommates since that is more of a specific kind of friendship. Friends are vague - everyone has friends. But, roommates are a special friend, most often, who you trust enough to live with, and in living with that person - the two develop a stronger sense of trust and openness not typically experienced in other friendships.
Bondage between roommates is a fun scenario because of that more complicated friendship. You're close to him enough so that you two decided to live together and share the bills. But that doesn't mean he doesn't have "quirks" that you find annoying. He over-eats even though you buy the food. Maybe he misses his share of the rent. Maybe he's not as organized as you are - or maybe his ocd levels of organization become a strain on you. In any case, you have to come to some agreement in order to continue living together. Well - it's hard for him to argue back when there's a sock stuffed in his mouth! And it's hard to take the sock out of your mouth and argue if your hands are tied behind your back. AND, it's hard to kick his ass if your legs are bound at your ankles and knees and tied in a hogtie!
3: Protagonist V Unknown AND Protagonist V Kidnappers
Now that I just got done speaking about how I prefer "Protagonist V Roommates" over Friends because Roommates provides more specifics, let me completely contradict that by saying I prefer "Protagonist V the Unknown" because I find the vagueness in the scenario most exciting.
Bondage between the protagonist and some unknown person(s) doesn't provide you with the same motivations that the other scenarios provide. Rather, it adds a mystery for the protagonist (and the reader) to figure out. Why did he attack me? Why chose me in the first place? What does he want? Bondage between someone and a kidnapper provides the answer to those questions and adds to the danger of the situation.