What reasons did President Obama give for not wearing the US flag pin?

First, this is another internet myth, based on some incorrect information. He did not "refuse," and it had nothing to do with his attitude about the flag. When then-Senator Obama was running for office, he didn't wear any lapel pins of any kind, nor did a number of other candidates. But his Republican opponents began saying the lack of a flag pin on Mr. Obama's lapel meant he was unpatriotic, a charge repeated on conservative talk radio as well. For a short time, he ignored the charges because he felt they were just politically motivated, but gradually, as more candidates began to wear flag lapel pins, and as his opponents kept questioning his patriotism, he decided it was best to go along with wearing a flag pin, so that it would not be a distraction in the campaign.