Mariam never got the letter from Jahil. She refused to go out when he waited for her outside her house in Kabul; he went back to Herat to die, and left the box with Mullah Faizullah, who in turn left it with his son Hamza. It remained unopened until Hamza gave it to Laila at the end.
The sadness comes from realizing how a small change in events - Mariam going out to see her father by his car, or Jahil letting Mariam into his house in the first place when she tried to see him - might have changed so much for her. I think Jahil was torn between feeling love for Mariam, and feeling the pressure of his culture and status, and was not strong enough to do the right thing, until it was much too late.
Jan, try to picture an aged Jimmy Page from Led Zeppelin, and a young female pop star in a zany outfit, in the middle of a giant stadium, doing a high pitched rendition of "Whole Lotta Love", a song known more for its driving rhythm than the poetry of its lyrics. It did not work well.
Steve