So... Losing Shiro is one of Keith's biggest fears, right? Why do you think he saw Shiro leaving him ("You chose to be alone") instead of... You know... Dying?
Because I think his deepest fear is more so linked to rejection specifically. Shiro suddenly disappearing or dying would probably prove entirely devastating and heartbreaking. But as for what he personally fears most, I think Shiro leaving him is far more accurate. When opening up about his feelings in his vlog, Keith specifically notes his abandonment issues and how they stem from his mother:
“I don’t know why I’m that way. Maybe, I’m naturally untrusting because my mom left me? And so, instead of accepting people into my life, I push them away before they reject me. I guess I have some walls up.”
The fact that he says “my mom left me,” and then immediately infers his tendency to distance himself from others is a direct correlation to this–it’s all very telling. “I push them away before they reject me.” His innermost fear here is rejection, a nightmare that echoes back to early childhood and memories steeped in desertion and starved affection. All this time, he’s actively cut himself away from others to avoid the eventual heartbreak. When arguing to keep looking for Shiro, Keith adamantly insisted that, “Shiro is the one person who never gave up on me. I won’t give up on him.”
And the notion that this one person Keith’s always trusted, the only one who’s never turned his back on him—the thought that Keith opened his heart and Shiro saw all of him but still chose to get up and walk away? The idea that the only one he could rely on, the person he loved most, would still just abandon him? That’s his worst fear.