Star Swirl strictly followed the path of a scholar to its final conclusion. His unfinished masterpiece (From one to another, another to one. A mark of one's destiny, singled out alone, fulfilled) represents his approach to life. To reach one's full potential, he believed isolation and training held the key. As his verse describes a solitary journey of accumulation, Star Swirl gained knowledge for his own sake. This granted him a wealth of knowledge about magic spells, but little about the magic born through bonds with others.
Just how Star Swirl represents the culmination of only seeking knowledge, Twilight reveals an alternative answer. As a pony who began following a similar path, Twilight would have reached a similar ending; however, the guidance of her friends and Princess Celestia sparked a change of heart. After experiencing the magic of friendship, Twilight continued magical studies while forging ever deepening bonds with her friends. Thus, giving birth to a new answer that Star Swirl couldn't conceive. "From all of us together, together we're friends. With the marks of our destinies made one, there is magic without end". Sharing their lives with one another has intertwined their fates- giving birth to magic that grows without limit: friendship.
Star Swirl attempted to create an ultimate magic with the answer found at his journey's end. Since he aimed to answer how to lead a fulfilling life and fulfill your destiny, the spell sets you on the path he followed. Singled out alone to focus on your goal without distraction. This made the Elements of Harmony a prime target since it symbolizes the unison of different elements/ponies to achieve something none could accomplish alone. While only the five pillars of the Elements of Harmony had cutie marks swapped, the citizens of Ponyville were emotionally separated and argued with one another. Twilight found her answer, which "A True, True Friend" embodies well. That connection between everypony is what makes Equestria such a magical place. It's what brings about that elusive sixth element.