Too many car puns in one tread, and people tire. They get flat. Eventually they exhaust. They drive line after line, then find it's not the same hub of conversation. They stop seeing the differential value of good and bad puns. Then maybe some fool injected an even worse one. It becomes like disease transmission. People start steering clear. They need a brake. They lose auto motivation.
Not in Spanish language train threads, though- they have loco motivation.