I love swimming for this reason. I've always been able to get into a weird "zen" state swimming, as far back as when I was a teenager on a swim team.
For some reason, I can't hit the same state with running, I don't know why.
I don't have access to a pool where I live now, so I've recently started running 3 miles a day. I still haven't reached the point where I enjoy it, it feels like work the whole time.
I'm hoping that as I build more endurance and skill this will change, but so far I just really miss swimming.