Joanie -
For security reasons, like almost any other website, your login expires after a certain time frame. This is for your own protection, so that members who login to a public computer and forget to logout do not leave their account and all their information available to any stranger.
We invite you to post an idea on our "Idea Board" to have a new setting built to allow members to stay logged in for a longer time period and see how many other members vote up that idea. Thanks.