Do Jin gets kidnapped -> Finds out it's his best friend -> Flashbacks to the past and of how he and Soo Gyum became friends through a certain "misunderstanding" -> Because Do Jin is a working adult (somewhere), Soo Gyum has to cover up for his sudden leave -> Visits the sister under the impression of "Do Jin asked me to give you money" -> When he visited the house, he flashbacks to when he realized he was in love with Do Jin -> He drives back home after declining coffee -> "That reality... I hate the most today." The fact that everyone trusts Soo Gyum so much is because he is Do Jin's "best friend" or in a sense his "brother".
Do Jin gets kidnapped -> Finds out it's his best friend -> Flashbacks to the past and of how he and Soo Gyum became friends through a certain "misunderstanding" -> Because Do Jin is a working adult (somewhere), Soo Gyum has to cover up for his sudden leave -> Visits the sister under the impression of "Do Jin asked me to give you money" -> When he visited the house, he flashbacks to when he realized he was in love with Do Jin -> He drives back home after declining coffee -> "That reality... I hate the most today." The fact that everyone trusts Soo Gyum so much is because he is Do Jin's "best friend" or in a sense his "brother".
HOPEFULLY IT MAKES MORE SENSE...