IPv6 only works after pinging the default gateway.

I gave the whole problem another try today, a couple of weeks later. And what can I say, I fixed it. Can someone please explain me why adding a ipv6 loopback fixed my problem? Here is what I've added to my /etc/network/interfaces file:

iface lo inet6 loopback

I have no ideas why I've forgot to add it in the first place!^^ Thank you all for your responses!