This is actually something you configure on your own computer. The FreeBSD command for doing it is ip6addrctl. I'm not sure what it's called on other systems. Also, note that the FreeBSD man pages are vague. You will also need to rfc-3484 to actually be able to use it. The man pages say it affects source and destination address selection, but they don't say how. It has two effects:
1. The command controls the sort used for DNS results. The default is to sort IPv6 addresses first. Properly converted applications actually get a list of IP addresses back from DNS and they are supposed to try them in order until the get one that works.
2. The command also controls which source address is paired with a destination address. This is less important, but something one might occasionally want to do.
For most ISPs in the US, it probably makes more sense to sort IPv4 first. At least for the next few years.