This is the 5979th most frequent Korean word.
삼았던
"삼았던" means "had considered" or "had regarded."
Here, '삼았던' is used to indicate that he considered or regarded someone as something, specifically his love, in the past.
그는 그녀를 사랑으로 삼았던 사람이다.
He was the person who regarded her as his love.
In this case, '삼았던' is used to describe the act of considering or treating a friend as family in past memories.
친구를 가족처럼 삼았던 기억이 떠오른다.
I recall the memories of treating a friend as if they were family.
Here, '삼았던' illustrates the act of taking or regarding a goal as something significant, like the centerpiece of life, in the past.