call me delusional but i like to think that the reason why sung hyunjae is so touchy with yoojin at the beginning of the novel is because he's testing his own boundaries.
this man isn't a fan of physical affection and doesn't initiate it normally (known from geunseo's QnA) but he does have exceptions. before han yoojin those exceptions were only song taewon and kang soyong. he obviously doesn't get touchy with soyong because she's a kid and they have more of a formal relationship where there isn't much reason for physical contact, and i doubt song taewon would let this man cling to him.
so physical contact is, in fact, rather foreign to sung hyunjae. it's not a thing he experiences often and something he's fine only with the small number of people he likes (stw and ksy), but there are still boundaries and stuff so he can't initiate it much and probably doesn't have the desire to do so (in stw's case i think he'd like to poke his brain more than his body, but i digress).
so, getting to my main point, sung hyunjae is testing his own boundaries with han yoojin. he can make excuses and reasons for touching him, he has the opportunities to do so, and han yoojin doesn't particularly object (he grumbles but he doesn't outright reject it and he doesn't look like he pays it any mind at all so there's no need to think about it seriously for either of them so say yes to testing things out without it being awkward (or acknowledged)). so since sung hyunjae likes yoojin, he wants to find out how far he can go (not necessarily in a romantic or sexual way, either. he's just experiencing things he didn't experience before and he's taking that opportunity to see if he likes it or not, and if he does, untill what point is it fine)
there is obviously the (not so) hidden motive of getting yoojin into his grasp and having him all to himself, but i think his curiosity also plays a big part in all this touchiness. he started off pretty normal and not too touchy with only occasional normal situations like intimidating a man in a car by holding him by the scruff; then the situations required him to carry yoojin, which, perhaps, he didn't find all that displeasing, prompting him to try and see if he's fine with physical contact when it comes to yoojin; then he starts opening up to yoojin more and more and getting more touchy: that time he hugged yoojin from behind and put his chin on his head when they fought in the mental realm. i think he did it because he understood yoojin more and they formed some sort of bond over fighting and grew quite close (it's not just the poison resistance okay!!!), and then when he found out yoojin wanted to rely on him more (it's not just because he shielded him from evelyn's arrows okay!!!), and then before his birthday when he decided to open up about his deja vu to yoojin (now this one really was unprompted and he wasn't forced into doing it by any Situations. he just did it. i have no possible excuses for that.)
so anyway i think he just wanted to find out how physical contact feels in general and had a convenient guinea pig under his nose, then he started liking yoojin and wanted to know how far he can go for that attraction, then he genuinely wanted to touch a person he likes more for the sake of it and because he likes touching him.
