I can't say 100% surely..... But from my POV I think namwoo just realized that sieon didn't know himself very well.... And that sieon might not completely in love with him but there was a possibility of something similar but because sieon didn't understand his feelings he just couldn't express him.... The reason I think those r because later on namwoo was like I know sieon more than he know himself (and then got jealous seeing sieons friend know sieon well too).... And also the fact that when sieon said "I love you" to namwoo, namwoo said he already knew.....besides I personally think sieon had some kind of feelings for namwoo at that time but he didn't realize his feeling because for other his feeling wasn't love.... And I think namwoo realized it and thought he just had to make sieon realize it too(hence he started saying "I love you" to sieon a lot.. Kinda like giving him a push)
Just my thinking I might very well be wrong though
can someone please please explain why namwoo (mc) came back to sieon (ml)???? he gave a whole speech but to this day, no amount of rereads makes me understand what his point was.