Timmy: (with rising anger and pounding his fist on the windowsill) I keep thinking if that firecracker hadn't gone off in Joey's face!

This line gives us the reason for Joey's blindness but the viewer wonders if Timmy was present at Joey's accident. We wonder if Timmy feels he "owes" something to Joey other than sympathy, friendship, understanding, patience, and kindness. We wonder if Timmy feels a guilt of some sort that induces him to part with Lassie.

Timmy: He just sits in that room and stares out the window.

Joey is blind and can't "stare" at anything.

Timmy: (having made the decision to give Lassie to Joey) Dad, there's just some things we have to do sometimes.
Paul: Yuh ... well, you do what you think is right, Timmy. (Gives Timmy one of those little man-to-man punches on the bicep then leaves the room with Ruth.)
Timmy: (burying his face in Lassie's ruff and crying with heartache) Oh, Lassie!