Why does iTunes want to delete my iPhone music?

[lo_and_behold]

I've added 1 new music file to iTunes that I created (not purchased) and tried to sync it to the phone but iTunes wants to totally delete/replace everything on the iPhone with the iTunes library which doesn't include everything that's already on the iPhone. Does this question make sense ?

 

[rambo47]

That's how all i-devices are set up. They associate with one library on a single computer. How did you get your music onto your iPhone? Was it with a different computer?