Seonho knows that outburst in the airport was a bad thing to do when he reflected on it. He had been bottling up those emotions for 5 months. Someone pointed it out that it's confirmed that Junseok was doing it on purpose. I'll quote them:
[ Remember that it's been 5 months of non-stop interruption from the glasses guy whenever they go on dates. If you didn't notice, it was demonstrated when Muyeong got a call from Junseok in ch 35, pg 27-29. The moment Muyeong mentioned Seonho's name, glasses guy immediately asked him to come visit. Muyeong even said, "But you told me it's fine to go out for dinner!" ]
He had been understanding the entire time until the point where they had to cancel their vacation because of a surprise call. What's good is that he knew he was in the wrong for being insecure over a person who was hospitalized. He's hurt and unsure of his position against someone who's been with Muyeong for many months.
Muyeong had attended to Junseok nearly 24/7, never even meeting with Seonho for two weeks straight and having to do his webcomic job all alone because apparently Muyeong had to QUIT HIS JOB because Junseok was demanding his presence in order to get better (never-mind how he's an incredibly rich man who can afford a lot of nurses and his family visits him).
We all know that Junseok is stubborn and that he likes to monopolise Muyeong's time to the point that he faked a suicide attempt in the most convenient timing. Even if he's depressed, that doesn't excuse you from emotionally manipulating a person and guilt-tripping them into visiting. (Ex. that scene where he wanted Muyeong to visit and when Muyeong said he can't always come, he was like "guess I'll die with no regrets then idc")
There is a thing called having a fucking conversation, I guess.
Muyeong's best and ONLY friend since childhood got into a huge accident, is paralysed, has to undergo therapy, and is depressed and perhaps suicidal. Ofc, his friend is going to be his priority and not a trip????
Did Seonho even once go to visit his friend?? Noooo.
No??? He didn't? It happened all in one day. The airport thing. Did you not see Muyeong thinking to himself in the car, saying 'what a day'. Seonho got mad at him, his friend confessed to him, his battery on his phone is low, his grandma visited him at night.
You honestly think someone who's angry and hurt on that VERY DAY gonna immediately change things?? Well guess what, Seonho did. Other people would've been more stubborn. Literally went all the way to Muyeong's house to TALK to him.
Look, sweetheart. Seonho is usually not the spiteful type. The only reason he's angry is that the shit has been going on for 5 months straight, purposely disrupting any alone time with Muyeong. Out of the two-- with puppy boy Seonho and stubborn, deppressed but arrogant Junseok. Who do you think would be the better man and visit? I have no doubt Seonho would have suggested to Muyeong to accompany him to the hospital. Junseok would definitely have said no to Muyeong and not let the guy in.
Actually, when you think about it. Seonho is an amazing dude to fix things on the very day they argued. If this were any other story, it would have dragged on for days or weeks. It was late at night, he had a revelation after the email-- he rushed all the way to his house because he wants to talk things out.
I know you said your opinion is based on the current chapters but I'll add this tadbit of info to disprove a point. In the raws (in the next chapter), Seonho was asking why Muyeong turned off his phone. Muyeong said his phone battery was dead. This means Seonho was trying to contact him on that very day.
is being a fucking bitch