If we take them separately, Makoto and Rin's backstories are already pretty traumatizing for a small child. Now if we look at them together we see that they have quite a lot in common: Both lost someone they cared about, Rin his father and Makoto the old man he was friends with, in a typhoon that sank their boat three kilometers off the coast. In episode 6 we see Makoto clinging to Haruka as a funeral part walks by and in episode 7 Rin is seen with Gou being part of one. Coincidence? I think not.