The answer comes in two parts -- one well understood, the other less so.
The first part is that young people are overwhelmingly supportive of gay marriage and, as they come of voting age, have begun to move the needle to their view. (In the latest Washington Post-ABC poll, a whopping 81 percent of those 18-29 think gay marriage should be legal.)
But, simply because young people are strongly supportive of allowing gay people to marry doesn't guarantee that the long-term trend on same sex marriage will continue to bend toward legalization. After all, there is the possibility that as young people age, they will grow less amenable to the idea of gay people marrying.
That's why this second chart -- and this second part of the explanation -- is so important.
What the chart above shows is that not only is each younger generation increasingly supportive of gay marriage but within each generation people are getting more and more ok with legalizing it as they get older. Just 25 percent of Baby Boomers backed legalizing gay marriage in 2004 but that number has rapidly risen all the way to 43 percent in 2012. Ditto Gen X'ers -- 37 percent of whom backed gay marriage in 2004 and 53 percent said the same in 2012.
Combine the fact that young people are heavily supportive of gay marriage and every generation is growing more in favor of legalization as they age and you see why the numbers on gay marriage have moved so quickly -- and why they aren't likely to ever reverse themselves.
Scott Clement, a polling analyst with Capital Insight, the independent polling group of Washington Post Media, contributed to this report.