system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How did the Western US and the North Eastern US both become majority liberal despite being on opposite sides of the country?

I was reading a New York Times article on Gay marriage and realized that the two "sections" of the country that are farthest away from each other seem to be the most similar politically. Is it something to do with their proximity to the water and therefore trading and immigrants? Just a thought.
gpt
Big coastal cities have more commerce and a stronger economy. More commerce means a higher population, and strong economies mean better education systems. Higher educated, more affluent people tend to lean more liberal statistically. They also tend to be less religious, and many American conservatives are deeply religious. Even in large cities in conservative states, they tend to be more liberal leaning than in rural towns. For example, a mostly religious and conservative city like Salt Lake City is considerably more liberal in overall attitudes than in small towns around the U.S. 

Edited: for clarity, redundancy.