Stig Anderson had assisted with the lyrics on prior ABBA singles - but on this one, it was all Benny Andersson and Björn Ulvaeus, establishing them as a strong songwriting team in their own right. This song is so sweet, and upbeat, that it endures to this day. Agnetha didn't have much of a solo career outside of Sweden (where she was a hitmaker BEFORE ABBA) but this is really her song, even though most of the lyrics are shared with Frida (real name: Anni-Frid - get it?).
By now, you should know that I like to include alternative versions when I make a simple post like this. This is the 1977 demo of the song. It speaks to the consistency of the band that it was hard to find any variation from the final studio version at all! Other than a slight key difference in the chorus, I don't hear one.
Compare this with a 1978 live recording..... in a lot of live performances, ABBA frequently lip-synced their songs - mostly because the studio recordings were so complex, and not at all an indicator . In this case - look in the background. There's a complete orchestra!!!
If you got this far - this song was famously covered by Erasure. Check out the Totally Covered post from 2012!