A human's desire was insatiable. This was a fact.
Just like a beggar, perhaps, his biggest wish was to just have a good meal and not to starve. However, when he was full and not starving, he would long for a better life…
Duan Ling Tian was the same. In the past, all he wanted was to improve his Innate Spiritual Root to a violet one with the help of the Divine Ability, the Elementary Devouring Tactic. However, after he had gained a violet Innate Spiritual Root, he began to long for a dark violet Innate Spiritual Root which was also supposedly the highest grade of Innate Spiritual Roots.
Initially, Duan Ling Tian thought this was the limit of the Innate Spiritual Roots, but when he found out that it was not through experimenting, and that it was likely there was a higher grade of Innate Spiritual Roots, he began to long for it as well.