Truth be told, many of the same things that are in relationships:
Good Timing - There used to be a time where songs, and songwriting, followed particular time signatures; 2/4, 3/4, 4/4, etc. Today's music, however, benefits from the lack of necessity of making the song fit the timing, but making the timing fit the song. We've become more intentional about getting our points across, musically, and doing so by presenting our original pieces unaltered and away from the beaten timing path. Good timing in relationships requires the same exact thing: intent. If you wait for time to get your point across to reach out, to check-in, to express, you may not have that time to do so when you're ready. Be intentional. Make timing fit your expressions.
Structure - Much like the timing issue above, there used to be tedious formula involved when it came down to songwriting: verse, chorus, verse, bridge, chorus, etc. Things, structurally, sounded the same. The structure is necessary: nothing grows without it, even and especially relationships. However, the structure doesn't have to follow a comparative outline, but simply the STANDARD: JESUS, WHO refused to put limits on how much HE'd do for us. Create your own structure that looks like HIS and not everybody else's.
A Great Chorus - The refrain, the hook, needs to catch people. What's being stated over and over, again and again, that we just can't ignore? Same thing with relationships, because whether we realize it or not, every relationship says something: about the relationship, itself, and the individuals involved in it, as well. What's being repeated about your relationship with others? Your relationship with JESUS??
Great Lyrics - The words. This part will ALWAYS be the difference-maker. The music is what it is, the production is what it is, but the lyrics??!!?? The words??!!?? What are you saying??? Lyrics are great because they never simply deal solely with where someone is now, but where they've been and where they hope to be. What are you saying about your relationship? The words, man. All words matter. ?
So what's in a song? A lot of the same things you can find in relationships. What's in YOURS???