Most of the time (not always) when I sing harmony, my objective is to make an invisible but not inaudible contribution to something I'm enjoying listening to i.e. to blend in a sound that contributes to what the lead singer is doing, without distracting from it.

The exceptions are with singers I know well, where we are used to harmonising a song which we know between us - and shanty singing, which is a different matter!

As ever, the song itself is what matters - it's just as easy to destroy a song with poor harmony as it is with inappropriate phrasing, decoration or whatever in the melody line. Call it "taste", "experience" or whatever - it comes down to being able to listen and learn....

Regards