How was David and Rosalind's friendship joyous and fulfilling at one point and sorrowful and burdensome at another in the Chrysalids?

I think for them it is always joyous and fulfilling, but it causes sorrow when David re-encounters Sophie. The relationship might be burdensome to her at that point, but I don't think that it becomes burdensome to anyone else, although of course it causes everyone sorrow to see Sophie suffering like that.