It isn't that Temari was obvious. It's more that Chouji is grateful for the talk and her friendship. He catches the aborted initial response and smiles at Temari when she accepts the offer rather than brushing it off.

"It's a shared kitchen anyway. I might as well cook for friends instead of just myself." There are plenty of reasons for Chouji to cook for Temari. Cooking for friends gives him more time to spend with them, keeps everyone from fighting for space in the kitchen at once, and lets Chouji do something nice for the people around him. It's a small gesture, but he likes making it.