Are you on a phone, tablet or pc/Mac?
If pc do you have cookies enabled?
Have you tried a different browser?
Giving more details helps people know what to suggest.
Until you figure it out you could simply use one tab. Or are you referring to each time you open the site?
If you close your browser you will have to sign in again each time. If you leave the site up for long periods of time without use it will log itself out.