You can't write your favourite song. Or perhaps, more accurately, I could say: you can't write the feel of your favourite song. The brain has had years (or months) to assimilate the 'information' of the song. You can't recreate that feel with something new. The feel that you want when you write a song is the feel you get when you hear a song for the first time and it gives you a buzz.
Perhaps as an aid to songwriting creativity you could 'flood' yourself with newness; listen to a radio and skip from station to station ensuring you only listen to songs (or music) that are new. Perhaps liking the songs (or even getting the 'new song buzz') is a bonus. Your ear gets a feel for melodic newness and this is sometimes (I've found) carried over to your songwriting creativity.