Itunes automatically transfer library to device?

I think this has just been happening since one of the more recent updates, but i'll have my music on my itunes, and when i plug in my iphone there'll be the music on my iphone under 'device.' but whenever i get new songs, i'd have to manually transfer it from my itunes library to my device area, and i tend to forget what music i'm downloading.

is there any way to change the settings so whenever i download a song, when i plug in my iphone it will automatically go from my library to device? And if so, how?

Should be a box that, when checked, automatically opens iTunes. If that is unchecked, it turns off several features. One of these is auto-sync.

If that's not the issue at hand then just check the settings, there may be something else unchecked (or checked) that shouldn't be.