Once, when Sam was very small, and Frodo wasn't all that much bigger, there'd been a kiss on the top of his head. He remembered the warm feel of it, although he didn't remember the occasion.
And later, when he was older, and Frodo was pretty much grown up, there'd been a kiss on his cheek, as he was falling asleep, and too tired from chasing after Pippin to even turn his head. He remembered that kiss too.
He'd thought about them both, and wondered what it would be like to have one more kiss. On the lips. Someday.