"Well, it's been building up inside of me for, oh, I don't know how long..."
If you know this record at all that line has probably been embedded in your memory since the first time you heard it, so perfectly do its seventeen syllables carry the tune. (Presumably he needed seventeen. That's why he puts an "of" between "inside" and "me".) In later years Wilson chased complication for its own sake. In "Don't Worry Baby" he was simply expressing in music the feelings of a young man whose heart was so full it hurt.
Wilson wrote the song with Roger Christian, a DJ who specialised in words for songs about cars. Presumably it's Roger we should thank for lines like "And if that ain't enough to make you flip your lid/ There's one more thing, I've got the pink slip, Daddy" in "Little Deuce Coupe". Christian could have come up with that first line. I doubt it. It wouldn't be hooky or detailed enough for a wordsmith. Wordsmiths read words. Musicians hear the sounds they make. Whenever a musician wants to bring another artist's song into conversation he'll quote it by singing a snatch of it. That's how he remembers it. A songwriter's job is to make sure you can never hear it any other way. With "Don't Worry Baby" Brian Wilson nailed it, as they say on The X Factor.