That is because you are syncing between two or more different computers, not the one computer you used to synced before. iTunes does not allow sharing or in your case, hoarding songs/videos/ringtones from different computers, every time you plug in your iPhone into a different computer, it will recognize it as a different syncing terminal and if you do choose to sync with that computer, it will delete all the previously synced media from the old computer, and use this new computer, with its contents as its content provider.

This old mentality of hoarding songs from your friends using China-brand MP3 players have to stop. If you need to do that, get a USB thumb drive and hoard away.