Characters
You're the one person with enough kindness and wisdom to understand this, but Fred, you're the one person I can never tell. Never, never. Because even if I waited until we were old, old people, you'd be bound to look back over the years and be hurt.
— Laura ‧ Brief Encounter

Behind the line May contain spoilers.
Our reading
An editorial interpretation.
Laura sees the very person most capable of understanding as the one she must keep in the dark. The fear is not limited to an immediate reaction: disclosure could change how he remembers years of their lives. Kindness and the wish to avoid hurt become painfully entangled.
Context reviewed