Why was Naomi very bitter?

The reason why Naomi told the women in Bethlehem, “Don’t call me Naomi  ..  Call me Mara”

Was not only because she went away full but she came back empty (Ruth 1:20-21)

But also her husband and two sons died (vv. 3, 5).

This means that she couldn’t have any grandson to succeed her husband.

I think that was why Naomi told those women to call her Mara.

I think that was why after the women said to Naomi, “Praise be to the LORD,

who this day has not left you without a kinsman-redeemer.  … 

He will renew your life and sustain you in your old age.  …” (vv. 14-15),

they said “Naomi has a son” (even though he was Naomi’s grandson) and named him “Obed” (v. 17).

“And Naomi took the child, laid him in her lap and cared for him” (v. 16).