Most of the time when i open up my gmail i have many new emails and which results as some of my important mails are kept unread and also goes back in queue when newer one overides them. Gmail by Google has many powerful features which every one does not knows them completely.
If you want to read all your unread emails in gmail there is a simple button which displays all your unread emails from the mail box. All you have to do is in the search bar type is:unread and hit enter. There are different functionalities available with is: command you can check it out.