That reminds me of a episode in Friends where Ross drove to the airport and met Emily at the gate. He told her he loves her and wishes to spend more time together and she said thank you before going on the plane. Then later, Emily calls him from US since she was looking for him and Ross was in England. She said she love him and he said "thank you." XDDD
I sometimes said thank you to my boyfriend. Though I don't think he felt content when I said that... Even though we have said I love you to each other for a lot of times for years, sometimes I just blank out and say thank you when I'm not focused in the conversation. '___' I want I love you to be said with feelings, not just to be replied back to.