In Alvin and the Chipmunks: The Squeakquel, they first meet in school and Alvin falls in love with her. While rehearsing, Alvin says "I never thought I'd say this, but... pink is my new favorite color!", he's obviously talking about after seeing Brittany, he thought she was so pretty, he joked about it being his favorite color.
In Alvin and the Chipmunks: The Squeakquel, The Chipmunks fall all over each other when rehearsing and smile happily while sighing, and all make comments about which Chipette they have a crush on.
In Alvin and the Chipmunks: The Squeakquel, Alvin warns Brittany about what Ian did to him and his brothers. Alvin's advice shows he cares for Brittany and doesn't want her to get hurt by Ian.
In Alvin and the Chipmunks: The Squeakquel, Alvin bumps into Brittany on the tray, causing him and Brittany to laugh nervously and get embarrassed.
in Alvin and the Chipmunks: The Squeakquel Alvin resolves himself to save the girls, saying "Britt, I'm coming to get you."
In Alvin and the Chipmunks: Chipwrecked, Alvin called Brittany "kind of the smart one" in response, she blushed.
In Alvin and the Chipmunks: Chipwrecked Alvin agrees that Brittany is the "pretty one".