Certainly a confusing relationship, with actions defined mainly by politics. However, the two's personal relationship may have at times caused or stood in defiance to interactions.
Despite Old Fritz' plans to marry Maria Theresa, he was forced by his adviser into marrying Elisabeth Christine, a marriage he despised every second of, declaring there could be no "love or friendship" between the two and even contemplates suicide before finally going through with the wedding. There have also been considerations by historians that Frederick the Great was a homosexual, with his relationship with Hans Verman von Katte in particular speculated to be romantic. This theory, if true, would further portray Frederick the Great's plan to marry Maria Theresa as a cold strategic move as opposed to being driven by any sort of affection.
As Maria Theresa was a woman, her father had to draft what is known as the Pragmatic Sanction to allow her to inherit his land. Many opposing nations wished to have some of the Austrian Empire's land, and so they opposed Maria Theresa's ascension to the throne. Old Fritz recognized Maria Theresa's ascension, but occupied the Austrian territory of Silesia and laid claim to the territory in return for aiding Maria Theresa and Austria against its enemies. If not ceded the land, Frederick the Great threatened to join those enemies. Maria Theresa, fearing that she would illegitimize the entire Pragmatic Sanction by giving in, defended the territory in what escalated into the War of Austrian Succession. In the end, Prussia kept Silesia and Maria Theresa began a life-long hatred for Old Fritz. A Prussian ambassador writes to Frederick the Great, " She detests Your Majesty, but acknowledges your ability. She cannot forget the loss of Silesia, nor her grief over the soldiers she lost in wars with you." She also referred to him as "that evil man."
Despite their obvious land conflicts and the supreme difference in their ruling styles (Maria Theresa was more willing to share power, less warmongering, but also a religiously intolerant Catholic to Old Fritz' autocratic, war-heavy, but religiously tolerant Protestant rule), the two did seem to share mutual respect for one another. A Prussian ambassador who wrote to Frederick the Great had many positive things to say about her, while Frederick the Great himself wrote of her, "That woman's achievements are those of a great man."
Certainly much of how Europe's territories were distributed and how wars played out depended on these two figures, and especially how the two interacted. Many have speculated how Europe's history would be different had Old Fritz and Maria Theresa actually married, though a good portion of that many believe that an Austrian-Prussian state would implode on itself due to the contradictory political nature of both nations. Should the two monarchs have hatred or admiration for one another, the two of them and their relationship have been very interesting and a fascinating topic for historians to this day.
Sources: Wikipedia, Departments.kings.edu, Mad Monarchist.blogspot