samba - Jan 2017 - Need to authenticate to Samba server regularly

I have setup a Samba4 server on CentOS7, and it's part of an AD domain.
Although I can browse to the samba shares on that server (from my win7
client), I notice that I regularly have to re-authenticate to the server.

 

I'm running Samba+SSSD.  I'm not sure which piece is responsible for
prompting for authentication, or what I have to setup incorrectly.  (Windows
servers on my domain don't ask me to reauthenticate like this).  Can someone
point  me in the right direction?