Keith is selectively mute by his own choice, due to bullying and mistreatment towards his stuttering problem, of course the constant bullying amde the stutter worse. (cause his fear that he will stutter when he talks makes him stutter more)
Keith is now in high school and hasnt spoken in several years, he now does what he can not to draw attention to himself.
Enter lance, who transfers into the school late, he runs into keith and is immediately offended when keith doesnt talk to him. Keith is mostly just shocked lance talks so much and is kinda secretly amazed lance didnt stumble over any of his words.
Time passes and keith, who is in the middle of having a bad day runs into lance, who probably stalks him with his self imposed rivalry. Keith not having the patience to deal with lance yells at him to "JUST SHUT UP" and that is enough to shock both lance and keith.
Keith then decides that lance somehow is the reason he didnt stumble over his words, his in the moment arguements with lance are the only times he doesnt stutter(probably because he is too worked up to focus on the fact he should be worried about stuttering)
Now keith begins to secretly adore lance cause he is the only person he can literally talk to without stuttering and that is a big deal to him, but lance on the other hand believes keith hates him cause they only talk when they argue or fight.
Mind you, the stutter isnt magically cured by love or anything... that would be nonsense...
And when keith finally gets the guts to talk to lance and confess without arguing, he is a blushing stuttering mess and lance finds that adorable.