My little one is doing this too! So exciting! She's 5.5 weeks, and doing it more and more now--it seems that whenever she's alert and comfortable, we can trigger it by staring into her eyes and smiling really big and talking in a happy voice--she also likes being "kissed" on the nose by her little stuffed dog, and when I "clap" her hands together (although they're still mostly in fists, so it's not really clapping). She's even added some "ahhhhh" sounds to the smiles sometimes. It's the best.