Taeyoung might have a strong fear of abandonment. I don't think his actions were complete possessiveness but a mixture of anxiety too. It's evident in the way Taeyoung questions why didn't Jinwoo come to him first and in the way Taeyoung obsessively marks him. Might also be the reason he only pursues casual relationships to avoid the intimacy that comes with serious ones. It could be due to his childhood where his mother might have "given" Taeyoung to his father to stay with him and essentially abandoned him.
Taeyoung might have a strong fear of abandonment. I don't think his actions were complete possessiveness but a mixture of anxiety too. It's evident in the way Taeyoung questions why didn't Jinwoo come to him first and in the way Taeyoung obsessively marks him. Might also be the reason he only pursues casual relationships to avoid the intimacy that comes with serious ones. It could be due to his childhood where his mother might have "given" Taeyoung to his father to stay with him and essentially abandoned him.