Midoriya Izuku did not understand his best friend, Natsuki Bakugou. She would bully him in many ways, sometimes by reminding him he was quirkless, on other times by calling him Deku, and she'd even tease him by showing off her quirk. Yet when others tried to do the same, she'd bare her fangs towards them and tell them to shut up or else she'd beat them up. He didn't understand why she hates having her mom hug her or even kiss her while she snuggles up to him when they have a sleepover.
He did not understand why he's allowed to call her Na-chan but the other kids get beaten up if they even try to. He didn't understand why when he tried to start calling her Natsuki instead she got mad at him and told him to go back to calling her Na-chan. He did not understand why she never listens to her parents, and teachers when they tell her to stop getting into fights but she listens to him.
Izuku didn't understand why despite Natsuki being so strong compared to him, she still stuck by him. He didn't understand why despite her bullying him a lot, she cared for him. He didn't understand why Natsuki even sees him as her best friend when he was a quirkless wimp that mumbled a lot. He definitely didn't understand when Natsuki just frowned (read: pouted) at him when he asked her this and just called him a baka deku.
Natsuki Bakugou did not understand her best friend, Midoriya Izuku. Even when she bullied him a lot, even when she teased him, and when she definitely hurt him a lot of times with her quirk, he never left her side. She didn't understand what was so different with Izuku that when she saw others try to hurt or bully him, she wanted to beat them up hard. She didn't understand why having her mom hug or kiss her annoyed her, but with Izuku she felt calm and serene as if she was just wrapped in the comfiest blanket ever.
She definitely didn't understand why she hated it when others would call her the same nickname she has for him. She didn't understand why she hated it when a girl would try to get close to Izuku and she would always have this sinking feeling in her chest. She didn't understand why her chest felt warm whenever she'd receive praise from him but when it came from other people, she never gave a damn. She didn't understand how he could make mumbling look so cute and how he could make smiling be the best thing ever.
Natsuki Bakugou did not understand why Izuku wanted to be a hero. He was quirkless, he could get hurt, she didn't want Izuku to get hurt. The idea of Izuku getting hurt by a villain scared her, she didn't understand why he doesn't realize it.
But there was one thing they did understand, they would never replace one another for someone else.