Indecision
The main character, Podkoliosin, is plagued by indecision. The fickle, arbitrary nature of his fleeting motivations ensures that he rarely does anything at all, and especially will never actually make the decision to do something as big as getting married. Without Kochkariov's interference, Podkoliosin would never have even met Agafya in the first place. This indecisive reluctance to accept responsibility leads him to desert his new fiancé at the end of the novel, jumping out a window rather than facing the (not unpleasant) consequences of his actions.
Agafya herself faces a crisis of indecision at several points as well; when pressured to make a decision between the men, she even breaks down crying. Her indecisiveness is nowhere near as severe as Podkoliosin's, but it is notable nonetheless.
Relational Interference (Matchmaking)
None of the action in Marriage would have occurred without the interfering activity of the novel's matchmakers, Fiokla and Kochkariov. Podkoliosin was lingering in his eternal state of playing with the idea of eventual marriage when Kochkariov comes along and forces him to take a concerted interest in Agafya, convincing him of marriage's benefits and preventing him from running away. Fiokla is similarly responsible; being the professional matchmaker in the story, she does the initial (yet deceptive) work of setting up Agafya with all her potential suitors, including Podkoliosin.
Rumor and Appearances
As the reader understands as the play progresses, when Fiokla (the matchmaker) was persuading her various subjects to develop an interest in the significant other, she tended to stretch the truth a bit, exaggerating certain elements of the person of interest in order to pander to her specific audiences. For example, to Yaichnitsa, who is motivated by material profit, she exaggerated the size and quality of Agafya's dowry. To Anuchkin, who merely wants a French-speaking bride, she exaggerated Agafya's fluency. To Agafya, moreover, she paints all of the prospective gentlemen in an appealing light, despite their obvious and irritating faults.
