If not already happened, add the following to /etc/udev/rules.d/70-persistent-net.rules:

SUBSYSTEM=="net", ACTION=="add", DRIVERS=="?*",
ATTR{dev_id}=="0x0", ATTR{type}=="1", KERNEL=="eth*",
RUN+="sysctl net.ipv6.conf.%k.use_tempaddr=2"

SUBSYSTEM=="net", ACTION=="add", DRIVERS=="?*",
ATTR{dev_id}=="0x0", ATTR{type}=="1", KERNEL=="wlan*",
RUN+="sysctl net.ipv6.conf.%k.use_tempaddr=2"

Usually this happens when installing miredo or similar IPv6 tunneling software, but you might want to double-check it. Also, check if the eth* and wlan* fit your ethernet/wifi devices.

btw, unfortunately Android and I-Phone Devices don’t have this, which causes quite a bit privacy concerns, see heise.de on IPv6 on Smartphones for more information (German)