It's not that people here are unfriendly, they will hold the door for you and wave you into traffic and stuff like that, it's that everything is maddeningly impersonal. The attitude is "have a nice day, somewhere else". It's easy to get along but making friends is almost impossible. People will say they want to hang out with you sometime and look at you like a freak when you actually suggest something. People enthusiastically say they are coming to a party then don't show up. People are flaky and hard to pin down. Girls lead you on for weeks and snub you with no explanation. People are insincere. Norms of social interaction don't apply here. Most people don't like or dislike you, they're totally indifferent. Every interaction will be maddeningly superficial.
I was a popular guy at my highschool in another state. I had plenty of friends, dated regularly, and was outgoing enough to make new friends easily at parties. I went to college in Seattle and didn't make a single friend for over six months. I would go weeks with nothing more than idle chit-chat. Friends from highschool less popular than me were off having the time of their lives in other schools. The next year (my sophmore year) I was in a dorm with a bunch of freshman athletes from other states. We hit it off instantly and became close friends. We threw our own parties, we helped eachother with schoolwork, we would hang out and just enjoy the company, I dated one of the hotter girls. It was a normal college experience, and only because there were enough people from out of town who actually wanted to make friends.
Every. Single. One. of the people I know who re-located here from another state will tell you it's hard to make friends here. They may not know the term "Seattle Freeze" but they well know the phenomenon. The only people who will deny it grew up here, and even many of them acknowledge it.