Ms. Callahan, who had recently gotten out of a four-month relationship, was less impulsive. She felt it was a good date, but preferred to take things slow. “I wasn’t thinking of this through a serious lens,” she said.

A spontaneous date happened on a bench at Washington Square Park a week later. More deep conversation ensued. Another date followed. Then another. Then it was Thanksgiving, which was spent together at Ms. Callahan’s apartment with each other’s friends.

“We were falling so naturally into a life together,” she said “I wasn’t taking time to reflect that maybe this was something serious. That’s when it clicked. My heart was in love but I wasn’t connecting the dots.”

In February 2018, Mr. Shah wanted to celebrate both Ms. Callahan’s birthday and Valentine’s Day and planned a surprise trip. Ms. Callahan was only told about the weather and to bring her passport. At Kennedy Airport, she learned they were flying to Sweden, and then to Denmark to see the singer Daniel Caesar perform.

“No one has ever done anything like that for me,” said Ms. Callahan, who cried at the airport once she found out where they were going. “That’s where I fell in love with him. Getting to know someone’s past and their life is special. I met his mom and his friends and got a complete picture of his life. It closed the circle for me. I know this person now and I love everything I’m seeing.”

While Ms. Callahan had her internal love revelation, Mr. Shah struggled to say his out loud.