Why is it that sometimes i sync my iphone some songs get deleted for no reason?

Sometimes when i sync my iphone (especially when i sync a lot of songs) other songs just seem to disappear from my iphone and i have to re-add them to my library to get them back on my iphone. Why does it keep doing this?

Make sure all songs are selected in your iTunes library and that you check to sync all songs under your iPhones settings in iTunes.