Well, first, you have to take someone who is already considered 'crazy'.... Someone like Woo Jin....
Well blondie did accept that he's indeed crazy (after Tae-min called him so). I think he was so distraught from Tae-min's death that he was willing to believe anything that pointed to Tae-min still being alive - even if that meant believing in the supernatural. I mean we've all heard about people who've sought out religions or cults after experiencing life changing event. The shock or trauma from the event causes us to find solace in weird places.
ok i understand si woon consciously attributing si won's personality to tae min... but how does anyone actually come up with the conclusion that he's actually him? like what kind of thought process does one need to have to actively say that out loud and believe you won't come out of that situation like you're not nuts ??