The reason he's so obnoxious is because he treats it like a competition. It's like it's less about getting his feelings across to the girl he likes and more about a smear campaign against the other guy. Just as Shinichi says he is a brat. He can't earnestly get his feelings across and be there for her he has to always bring the other guy down and tar and feather him.
Yeah. He can't seem to accept that he's betting on a team that's down by 100 in the last minute of the game; there's some tiny miracle shot they can win but, realistically they've already lost. Utako and Shinichi have already tried to get through to him, but I think it's gonna take Haruka finally getting sick of him trying to play interference and snapping at him for it to finally sink it that this just isn't going to happen no matter how annoyingly aggressive he gets.
I used to like Maa-kun and I actually felt sorry for him and his unrequited love. It even looked like he learned his lesson and went through some real character growth after Utako confessed to him. Now he's getting on my nerves. Haruka has rejected him several times and he broke off contact with her for years and now he's acting like a childish, selfish, and arrogant ass. He's very aware of how uncomfortable this whole situation is for her and how much she doesn't want to break his heart and is taking advantage of her feelings. He's trying to get her to doubt her feelings, force a confrontation that'll break her and Shinichi up, and then pick her up on the rebound while she's confused and depressed. That's not how you treat your friends and it's certainly not love; that's being completely fixated on someone and being obsessed with not losing to someone else. If he truly loved Haruka, he would want her to be happy even if it meant her being happy with someone else, like how Satomi acted with Shinichi. I sincerely hope he decides to come to his senses before he does something he can't take back. I also hope that Shinichi can get there before Haruka's mother does something awful to her daughter or to herself.