Love is a concept represent the deepest affection a individual reflect to another. To the point of desire the good for the latter over the one himself/herself, hoping to build a relationship with the loved, and the urge to maintain their bond.
Tgurneu, being one of the leader of Kyouma and an existance who can't feel love himself, had develop a fanatical faith regarding the feeling that both human and Kyouma shared. In his own belief, beings whom love can obtain happiness and strength that achieve deed that were consider impossible under normal means. Therefore making power of love the greatest in the world.
Despite being a positive feeling itself, it can be programmed by ability that Tgurneu gained without the realize of the subject, as both Saint of the Single Flower and Adlet Mayer fell as his victims. They will later conduct dire actions that could have destroyed the world they should have saved under the urge of false love
Basically, human love each other and their own offsprings while Kyouma born to love Majin with their loyalty except Tgurneu himself. However, only two known figures had overcome impossible odds with love by rivaling Braves of the Six Flowers without natural bressings those chosens have.