Cannot resist the ficathons. Hermione didn't cry when she sent her parents away. Ron didn't cry when they fled his childhood home under attack.
Harry, well, Harry had thought he was out of tears in a cupboard under the stair, but then there was Sirius, and moments concerning James and Lily, where they found him again.
After all that not crying, Ron left, and things fell apart.
Hermione sobbed when Ron left. Ron sobbed when Hermione was tortured. And Harry sobbed as he buried his most blindly loyal supporter, Dobby.
The end came. Harry and Hermione both found that their tears were broken. Tears had been suppressed and saved then spent in bouts of fury and pain, but there was no familial pool to replenish them from, no return to the source of life and love and emotions to fix the broken tears. The loved ones were gone, no one left to shed tears with, only people to shed tears for.
Ron found tears for Fred, but Harry nor Hermione could find tears for Fred or Remus or Tonks or their beloved home of six years or the students who perished within its walls. Harry rationalized that when you had recently died yourself and shed no tears for yourself it was unsurprising to lack tears for others.
Anyone who even noticed assumed that Harry and Hermione slipped in the back at the funerals out of respect for the families of the dead, to refrain from detracting attention from those who sacrificed so much. A few well-placed, fashionable hats and the picture of public sorrow and mourning was complete, so that's what people thought. Truthfully, it was that they could not - and could not want to - cry. But together it was safe to be broken in all the same ways.