One of the most annoying thing about CentOS 7 (actually, one of the very few annoying things) is the default configuration involving systemd journal logs.
The default is indeed to clear the whole log on every boot.
If you’d like to preserve the journal (to being able to see what happened before the system reboot) you just need to edit the file /etc/systemd/journald.conf and in the section [Journal] change the value of the Storage entry from auto to persistent.
[Journal]
Storage=persistent
That will do the trick!
You just need to restart the journald service in order to make the new setting effective:
systemctl restart systemd-journald