As a rule, my email "inbox" is for mail that I need to do something about. I may need to respond to it, or it may contain information about an upcoming event that I want to have handy. But I've recently discovered another category: email from people who make me happy. It turns out, when someone surprises me with an email after a long silence, or if it's someone I have a crush on, or someone whose presence just generally makes me happy, I leave their email in my inbox even if I could "file" it because seeing their name in the list makes me happy.
Gosh, I'm weird.