If you follow politics with even a passing interest, you know by now that support for gay marriage is surging -- even as the Supreme Court prepares to take on two major cases dealing with the subject.

What has been less obvious from the scads of polling data on the movement toward legalizing gay marriage is the "why" behind that movement. As in, why, over the last decade, has public opinion changed so rapidly on the issue?

Thanks to Pew Research Center data, we have some answers to that question.  The organization's mid-March poll found that 14 percent of people now supportive of gay marriage said they had changed their minds on the subject.  Pew followed up with those mind-changers to ask them why.  Here's a chart detailing what they said:

The most commonly cited reason for the change of heart -- offered by one in three respondents -- is that they know someone who is gay.  Interestingly, that's the reason Ohio Sen. Rob Portman gave for his decision to come out in support of gay marriage recently; Portman's son, Will, told his parents two years ago that he is gay.

Other regularly-mentioned reasons for changing opinions on gay marriage include "grown more open/thought about it more" (25 percent), "it's inevitable" (18 percent) and "everyone is free to choose" (18 percent).

Doc Sweitzer, a Democratic media consultant based in Philadelphia, offered another take on why attitudes on gay marriage have shifted. "Here's the answer: Television," Sweitzer wrote in an email to the Fix. "It's the greatest socializing tool of all time. Archie Bunker changed attitudes about race and the generational divide. Gays are portrayed in all kinds of shows in a positive light, from 'Law and Order' to 'CSI'. They are shown as people who just want to live their lives."

Whatever the reason, sentiment is quite clearly shifting. And, as we have documented in this space, the fact that a) young people are far more supportive of gay marriage than older people and b) each generation gets more supportive of it as it ages suggests that the political debate over gay marriage is likely over.