A short conversation can keep unfolding in thought long after its speakers separate.

Exchanges do not end when people walk away. A sentence that seemed unremarkable at the time can carry on working for days, opening slightly, attaching itself to something else you were thinking about, until you find you have changed your mind without deciding to. Neither party knows this is happening. The person who said it has usually forgotten by dinner and would be surprised to learn it mattered.

Since you cannot tell which remarks will do this, be a little careful with the offhand ones. And if something someone said has stayed with you, tell them, even months later.