어서

Because... (often trailing off)

Gives the reason for something and, in these lines, frequently ends the sentence there and leaves the conclusion unsaid: I was wondering whether you felt shy, so...; everyone's busy with studying and work and housework, so...; because it's just Ji Seok Jin? That unfinished shape is a soft way of explaining yourself. It also simply chains two actions, as in folding a paper crane and then confessing. Unlike 니까, it cannot be followed by a command or a suggestion.