The only thing that perhaps enforced that insecurity in Brooke (other than Peyton declaring her feelings) was Lucas going away with Karen for a little while. But that was a stupid thing for Mark Schwahn to have Brooke be jealous about; his mother was grieving and needed a vacation. That had nothing to do with Lucas shutting Brooke out.
His first words when they're driving back is "I'm thinking about Brooke" when Karen asks him.