In the long run, he makes the situation between the two families better, even though his plan hurts them very much.
If Friar Lawrence had not married Romeo and Juliet, the two of them would, presumably, have stayed alive. This would surely have made their families feel happier. But the two of them surviving would certainly not have made the situation between their families any better.
I think that the two families really needed a shock to shake them up a bit and make them understand how stupid their feud was. The deaths of the two supplied that shock. Therefore, marrying Romeo and Juliet led to their deaths and that led to the families reconciling with one another.
Ultimately, the actions of Friar Lawrence make the feud between the families better. By the time Friar Lawrence is even made aware of the feelings Romeo and Juliet have for one another, the "dye is already cast" regarding their fate. We sense that Romeo and Juliet will act impulsively to be together with or without the Friar's help; Friar Lawrence's action may have only increased the speed with which that was able to happen. Without Friar Lawrence, Romeo and Juliet would have likely found other--perhaps even more reckless and impetuous--ways to be together.
Also, by the time Friar Lawrence is made aware of Romeo and Juliet's feelings, Tybalt is already full of the "bitter gall" that drives him to confront Romeo in the street. Likely, a fight between Romeo and Tybalt was inevitable and the consequences of such a fight were most likely deadly in one way or another.
As unfortunate as the final outcome is for Romeo and Juliet, without Friar Lawrence's actions, the feud would have likely raged on. In part because of Friar Lawrence, the feud is brought to an end.