Some friendships form through shared experiences at school, others through a soccer club or workplace.
For Jan Wolfenstädter and Tina Kunath, their bond began with her diagnosis.
Kunath was 8 years old when doctors found she had blood cancer. She needed a stem cell donor and found one in Wolfenstädter.
He also became a lifelong friend.
Their friendship is extraordinary, say Tina Kunath, now 21, and Jan Wolfenstädter, now 34 years old.
"You realize that you are connected in a different way," Kunath told dpa as she sipped a mint tea in Cologne. Wolfenstädter sat opposite, nodding as he nursed a double espresso.
They recently met up at the DKMS, an organization that combats blood cancer.
The group finds an average of 23 stem cell donors per day in Germany, it says.
Including for Wolfenstädter and Kunath, who are now celebrating a decade of being friends by visiting Cologne Cathedral, the city's towering landmark.
Wolfenstädter was living in Berlin when he received the phone call in 2011. He was in the middle of training and prohibited from checking his phone but he recognized who was calling.
The call came from the city of Tübingen where a DKMS centre is located and Wolfenstädter had registered as a potential stem cell donor.
He took the call. "It was a very short conversation," he says. "Essentially, it was about what I was doing next week."
He knew what needed to be done.
Those were bad times for Tina Kunath, who comes from near Köthen in Saxony-Anhalt. Chemotherapy was not proving as successful as she had hoped.
She was spending her life in an isolated room. "Even as a child, I realized the seriousness of the situation," she recalls. Once lively, now, life was excessively quiet.
Then came the news that a potential donor had been found.
"I knew then that this was probably a new chance for me to get better," she says.
In the form of blood cancer that she had, the haematopoietic stem cells are defective, meaning fewer and fewer blood cells enter the bloodstream, which can be life-threatening in the long term.
A stem cell transplant means a donor provides healthy haematopoietic stem cells, that are then used instead of the recipient's defective stem cells.
Wolfenstädter and Kunath say they didn't really know who the other person was at the time.
That changed in 2014 when DKMS launched its first World Blood Cancer Day (WBCD) to raise awareness of blood cancer and stem cell donation.
That was when Wolfenstädter and Kunath met, and from then on, their lives became intertwined.
Wolfenstädter says that when he made the donation, he already knew it could potentially save someone's life. "But it only really dawned on me when we got to know each other. When I saw her face."
He remembers exactly how the donors and recipients met at the event. All he knew was that it had to be a young girl, so was looking downwards rather than up. It did not take him long to find her. "We knew immediately," says Wolfenstädter. "In a second."
It was not just a single encounter. Tina Kunath and Jan Wolfenstädter are not only "genetic twins," as the DKMS calls them, based on the stem cell donation - they also really like each other.
As their friendship grew, they began visiting each other regularly.
Kunath recently travelled to Berlin, where Wolfenstädter works for an aircraft engine manufacturer.
They celebrate each other's birthdays and talk about music. Today, they chat about cooking.
When asked to describe their friendship more closely, Kunath says Wolfenstädter is like an older brother.
His description is similar. "It's friendship, but it also has a family component," he says.
Perhaps it is the kind of connection you can only understand if you are part of it.
Separate lives, different family trees, but joined at a cellular level.
Kunath is now studying law in Halle (Saale) and is considering becoming a lawyer. She also skis regularly, a passion even before she became ill. She plays tennis and is often outdoors. "Luckily, I can do all that again now," she says.
Kunath is considered cured and there is nothing quiet about her life.
Not least because she and Wolfenstädter have expanded their friendship by heading to heavy metal music festivals. "I just took you there," he says to her. "It was great too."
