How to sync music between MacBook Pro and iPhone without losing the current library

I have a huge library of music on my MacBook Pro and my iPhone that were all synced. When adding additional songs or albums to my MacBook it seems impossible to just transfer the newer music to the iPhone without totally erasing the whole iphone library and reinstalling the whole library. Couldn't Apple have designed a way to do that. Seems so redundant to me. And why when you bring up the library in the Mac and right click on an album or song and get the option to add to iPhone device it does nothing.

[Re-Titled by Moderator]