** I think this is going to be the last "NCT Dream" content that I put Mark in ☹️ because whether I like it or not, he did graduate.... Enjoy! **
Mark wouldn't have a problem with you being friends or talking with the other members. He honestly thought it was cute and really valued that the other members approved of your relationship! Mark isn't the super jealous type, but like any other person, some situations would upset him and cause him to become possessive/jealous.
You and Haechan had grown closer over the past few months. You being with Mark and him being Mark's best friend you had a lot in common and a lot to talk about! Mark thought it was great that you were close, and it made going out as a group a lot more fun. But.....he started to think that maybe you two were getting a bit too close.
Haechan is obviously a big jokester and you had your playful moments too. So, when Mark walked in on you two play fighting, he couldn't help but get a knot in his stomach when noticing how physically close you were; and judging by the laughter and smiles on your faces, you two didn't mind. He cleared his throat in order to get your attention. You and Haechan turned to look at him, still laughing, while you had Haechan stuck in a headlock. "Hey babe!" You'd say as your smile grew upon seeing him "Hey Mark; your girl/boy has some killer muscles" Haechan would laugh as you continue to playfully shove one another. Mark wouldn't want to cause a scene, but he felt hurt. He didn't know how to take the situation and his mind immediately thought that the two of you were flirting with one another. He calmly, but clearly upset, walked over to you and pulled you (gently) out of the room by your arm. "Mark? What's wrong? What happened??" You would ask but he ignored you until he finally found an empty practice room to take you into. You'd ask him for the 10th time what was wrong and finally he sighed. He'd be quiet and probably say he was sorry at first. He never raised his voice at you and he was always patient and calm during arguments. But you could tell that right now he was pretty heated so you went silent and gave him the opportunity to speak his mind.
"Is this relationship a joke to you?" He would ask very seriously.
"W-What Mark I-" you would try to refuse and deny that but he cut you off
"Why would you two flirt like that? Especially here when other people can see you. When I can see you?! You know what people will say?! If you like him then why the hell are you with me?!" He raised his voice a bit
"Mark....I don't know why you are thinking like this. You know Haechan and I are good friends! I would never flirt with him! I would never do that to you!" You stepped closer to him and rested your hands on his shoulder. His head was hung low so you couldn't see what his face looked like
"I know....I'm sorry....it just kills me seeing you like that..." he whispered and your heart broke
"No....I'm sorry Mark. I didn't know you felt that way. Haechan and I won't do stuff like that anymore I promise! Okay??" You bend down and look him in the eyes. This whole situation clearly upset him. He wasn't super angry or anything; he was just scared he would lose you. It didn't cause a big argument or anything and you worked it out that day as soon as he opened up about how he was feeling. This problem actually made you guys grow closer! You were happy he opened up and he felt more comfortable saying what was on his mind. You made sure not to give too much attention to the other members, but you were all still friends!