Why because simply people look at the headline to see if the rest of the story is relevant to what they are looking for and are interested in. You should remember that the headline is like a small advertisement to what the rest information is about.
A simple way to test this is to look back through the emails that you have received in the last 48 hours and check which ones did Iopen and read, which ones did I leave for another day and which, ones did I delete without checking.
Once you have done this you will be on your way to working out headlines, because you will see the ones which you read were the ones with the most inviting headline that attracted you in to read more.
Armed with this information you can then work to plan how YOUR emails should be setup to target the audience that you are looking for to encourage them to read your emails now and not send them to the delete folder.