Taylor Swift - Our Song Lyrics Meaning

This is about a girl (Taylor presumably) and her boyfriend. When she suddenly realises that they don't have a song, like most couples ("I say nothing, I was just thinking, how we don't have a song").
When she says this, he basically says that they don't need one because 'their song' is all those things she talks about in the chorus - it's all the small things that makes their relationship so special ("the slammin' screen door, sneaking out late, tapping on your window, when we're on the phone and you talk real slow ... the way you laugh, the first date 'man I didn't kiss her and I should have'" etc. etc.). In the second verse, she says how their song cheers her up after a really tough day ("after everything that day"), and in the 3rd verse, how she can't find anything as good or as special as 'their song'("heard every album, listened to the radio, waiting for something to come along, that was as good as our song"). Finally, she writes down their song ("grabbed a pen and an old napkin, and i wrote down our song")