"An invisible red thread connects those who are destined to meet, regardless of time, place, or circumstance. The thread may stretch or tangle, but it will never break.”
I honestly believe that Mako’s scarf represents the string of fate. The scarf, the last thing left of his father, his most treasured possession, brings him peace, Korra brings him peace. No one has ever wore that scarf except for Korra. She won his heart and healed the wound from his parents death. She brought down his cold demeanor, lit up his soul and brought him comfort and love. The red string of fate is tied around these two lovers and no matter what happens in the future, they are meant for each other.