The web app may be logging you out when you close your browser or the desktop app may be logging you out if you don't use it for a while.
You can fix this in the web app by ticking a box during login, and in the desktop apps you can disable the Automatic logout option under Preferences.
Please note that if you decide to do this, you should be careful to log out each time you leave your computer.
On the web app
When logging in to your Dashlane account in the web app or in the extension, you'll be given the opportunity to tick a box that reads "Keep me logged in for 14 days." This will ensure that your Dashlane vault remains accessible in your browser (even if you restart the browser) over the following two weeks, or until you manually log out.
In the Windows app
Select the Tools menu in the top bar menu, then Preferences.
Go to the Security tab and click the lock icon at the bottom to unlock the window.
Now uncheck the Automatic logout option.
In the macOS app
Select Dashlane in the top bar menu, then Preferences.
Go to the Security tab in the preferences and click the lock icon at the bottom to unlock the window.
Now set the slider to Never in the Automatic logout section.