(In case it's not a given, spoilers for Days of Future Past below.)
He didn't. He just has all his memories from the original timeline (e.g. previous X-Men and Wolverine films), plus the events he experienced during Days of Future Past. He has no memory of what happened to him between 1973 and the present in the new timeline (which he created). That's why he's surprised to see a few characters at the end of DoFP. He remembers them from the original timeline, but he also remembers them dying in the original timeline. What would be much stranger is if they had never met him in the new timelines (in which case he would remember them but they wouldn't know him), or if he had met someone in the new timeline but not the original timeline (in which case they would know him but he would have no memory of them).
He didn't. He just has all his memories from the original timeline (e.g. previous X-Men and Wolverine films), plus the events he experienced during Days of Future Past. He has no memory of what happened to him between 1973 and the present in the new timeline (which he created). That's why he's surprised to see a few characters at the end of DoFP. He remembers them from the original timeline, but he also remembers them dying in the original timeline. What would be much stranger is if they had never met him in the new timelines (in which case he would remember them but they wouldn't know him), or if he had met someone in the new timeline but not the original timeline (in which case they would know him but he would have no memory of them).