5s deletes itunes library when i add 1 song???

Need help - been going on for a while now. I use an Imac to connect to my iphone 5s to upload songs onto itunes on 5s. I never sync. Manual upload only.

Heres what i go through:

Playlist 1 is already on my iphone 5s (every song on my Imac itunes is)

I want to add a new song to Playlist 1,

i update the playlist

and now move it to my 5s.

Now my 5s deletes the ENTIRE library just to re upload the ENTIRE library.

WHY!!!!?????