It seems that the Stoics were prepared to say two things : 1) It will in fact pay to be virtuous provided that you want to be happy; and 2) the good man will seek virtue for its own sake. Some of the apparent difficulties in reconciling these propositions may be resolved by examining the notion of seeking virtue for its own sake. What then do the Stoics say that virtue is?
Any Cynic could advocate a consistent life, for the description is purely formal. But one consistent life might be set against another, and Zeno's appeal to natural consistency prevents this, as well as showing exactly why virtue pays. The question could, of course, have been tackled in another way. Is there in fact more than one kind of consistent life? Zeno would certainly have agreed that there is not, but though he thought that in all but the wise inconsistency leads to conflict and misery, he did not ask such a necessary question as: Is an injury to someone else also an injury to myself? Why did he not? In part because by separating the goal (happiness) from the end (virtue) he underestimated the importance of eudaemonism in preaching a moral system to the unconverted. Or if he did not underestimate it, he kept implying that he did and that one should.
Rist, John M., "Zeno and Stoic Consistency" (1973). The Society for Ancient Greek Philosophy Newsletter. 446.