The real struggle of love is always caused by that there is too more than which one can accept, but not too little that one have given.
Since it is reasonable to ask for more love, whereas it is impossible to stop somebody from loving you without, sometimes even, hurting him/her. Hereby, it comes the unresolvable eternity among lovers in triangle relationship. Because good wills will not only be unable to bring everyone to a happy ending, but also pushing everyone to the inferno of more emotional agony. For this reason, romance becomes one of the greatest main theme in human literature since the vey beginning.
Because of the geometric trait of a triangle, the stability of the shape is unbreakable. The opportunity to end this kind of struggle can only come from external. As the shape has to change by either the elimination of one side or the addition of a new side. The former will give people a comedy; the latter would always manifest itself as a tragedy. However, since the addition of a side require the simultaneous contacting and connection to two sides, and the power of pry the joint open, it is much harder than to break an existent side and le the other two left coincide by themselves. That is why there are more great tragedies than great comedies in the history of romances.