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April 15, the day of the Boston Marathon bombings, was one of the worst days of James Bim Costello's life. But now, he says, he realizes why he was involved in the tragedy.

Costello first saw Krista D'Agostino at Spaulding Rehabilitation Hospital, where she is a nurse. He was there recovering from multiple surgeries for burns on his arm and leg.

"After sharing a handful of conversations, I realized not only how beautiful she was but also what a kind heart she had. I somehow convinced her to attend a benefit with me, which turned into a few dates, which turned into a few inseparable months," Costello posted on his Facebook page.

Now, just eight months after the bombing, he popped the question in Lyon, France, while on trip to Europe on a "Heroes" cruise for bombing survivors. She said yes.

Costello posted: "I now realized why I was involved in the tragedy. It was to meet my best friend, and the love of my life."

The couple is due back in the United States on Dec. 21.