can't sync music, "Sync Music" has become unticked in iTunes

I plugged my iphone6 ios8.1.3 in to iTunes 12.1.2.27 in order to add some new songs

however

the Music > Sync Music option has become unticked and shows no content underneath as a result

if I select it it says it will wipe existing content

given that I have about 50gb+ of music on the phone already, I really don't want to have to resync the entire library again as it takes a really long time

is there a way I can trick iTunes into thinking it IS indeed set up to sync?