Typically that happens when you do not have cookies in your browser from a previous session, or else when your network IP-address changes.
If your browser is not set to erase cookies when you close it, and you are not using a cleaner program that deletes your cookies for you, then some other things that can cause that are using a VPN, a hotspot or a TOR browser. Certain kinds of privacy software that hide some of your browser or network info can also cause issues for some users.
Sometimes an eBay cookie can become corrupt; in that case manually deleting just your eBay cookies once and signing back in may fix the issue, though you will have to go through Captcha after deleting your cookies.
