What keeps you here?

How do I make a decision where to live? I love my life in SF...right now. I could see being much less happy were certain cool people to graduate or go back to school or find different jobs. Or possibly more happy were certain ones to relocate here too. It works both ways. I can't make a decision based off of friends, right?
And yet, that somehow becomes my argument for returning to Portland. I've already got the friends, the know-how, the activities to do. There's enough new to explore but the comfort of the old, and the housing market is ripe for a hostile takeover. This is also a decent argument for going to Boston, or trying out Austin (whoa 100+ degree temperatures might be a big no on that one). So why change? Why move? And when? I do want a house...eventually. Maybe I don't actively want one now, but I could take on the responsibility right now. Should I do so just because I can? Should I change just because I've gotten comfortable?
Let's not even pretend this is about my career. I like my job. It affords me an excellent life full of travel and soccer and friends and being active and outdoors and eating good food. That's what I like about it. I have to work at being ok with that sometimes, but I'm ok with that.
Maybe I'm just uncomfortable not having any more goals to have. I don't really need the house, I'm ok without the man, I want a dog but that's still in the future. The job seems to be running itself. So now what?
well...
in my case, I think about my options regularly but only make changes when I wake up one morning and realize its time. In college I knew I wanted to live overseas so I started looking at my options before I was ready to actually leave so that by the time I was ready to leave, the groundwork had already been laid. When I got back from Australia, I moved back in with the parents to save some money (I was really, really broke...) and it was nice to see the fam again. But I had always wanted to live in one of those big, old houses in NW so one day I decided it was time for a change and within two weeks I was renting a room in a house. But even while living in NW, I knew that I eventually wanted to move to the east side and I wanted to try living by myself at some point in my life. So one morning on the way to work I realized that if I saw one more girl with a tiny dog wearing a tiny coat sticking out of her purse that I would have to run her over, so it was definitely time to move to the east side. Within two weeks, I had my cute little one bedroom apartment in SE. I loved my apartment but knew that I eventually wanted to have my own house and that I would feel like a badass if I did it while still in my 20s. But like you, I had a lot of hesitation about what owning a house means so for a couple years I didn't worry too much about house shopping until one day I woke up and realized that 550 sq ft just really wasn't cutting it anymore. And that day I started looking at houses. Even now, though I love my house and I'm happy there, I know that it's just a starter house and probably sometime in the next 5 yrs I'll want to move to the next level (preferably somewhere with less coverage in the evening news...).
Basically I've found that my subconscious will usually tell me when its time to change so I don't worry too much about putting a deadline on it. Instead I just casually think about my options so that when I wake up one morning and realize "Its time" I have already consciously (or subconsciously) decided on the next step. And obviously moving from one side of Portland to the other is not even close to being as big a change as moving cities (new apt, new job, new friends), but I think the same rules can still apply. And of course there is also that other part of my brain that thinks I should just throw a toothbrush and clean pair of underwear in a backpack and live with the yaks in Nepal for a while. Sometimes it makes me sad that that part of the brain hasn't won the majority vote yet, but in general I think my subconscious has done a fairly good job of leading me down the right path.