The sefer Nachalat Yaakov comments on the famous Midrash that when Yaakov Avinu had the dream about the ladder,
“He took from the rocks of the place,” and it ended up as one big rock.
Most people have heard that the rocks were arguing with each other, because each one said,
עלי יניח הצדיק את ראשו
the Sadik’s head should rest on me, and they became one rock.
The Midrash adds that Yaakov Avinu said that Avraham had negativity come from him- he had Yishmael, and Yitzhak had negativity as well, he had Essav.
But when the rocks came together, Yaakov knew that no negativity would come out from him.
What does that mean?
He explains that Yaakov Avinu understood prophetically that there would be two divisions in the Jewish people- Yehuda and Yosef.
And in time, the two tribes would split into two groups in Yisrael. Yaakov Avinu was afraid that eventually, they’d be lost, because when there is Machloket/arguments the Jewish people can’t last. So Yaakov Avinu wanted to see if the rocks would come together, and if they would ultimately unite. And when he saw that, he knew he’d have the Jewish people.
He adds that
עלי יניח הצדיק את ראשו
Upon me the Sadik should rest his head indicates that ultimately, the Jewish people’s arguments were L’Shem Shamayim. Each group wants Hashem to be with them. And that is what a lot of our fighting is- it’s religious fighting; but in the end, we will all unite.
The Shulchan Aruch brings down an interesting custom that people put a rock beneath their heads on Tisha b’Av. This is sourced from the Rishonim that say that Yaakov Avinu was at the Bet HaMikdash the night of his dream, and he saw that the divisiveness of the Jewish people caused the churban/destruction and therefore, he put a rock under his head.
In order to commemorate that act of Yaakov Avinu placing a rock under his head, we do the same (or as some do, sleep without a pillow) on the night of Tisha BAv.
The point is to realize that what brings the destruction is a lack of unity, and what will rebuild the Bet HaMikdash is when we all get together as one rock, and we say together, with one voice,
עלי יניח הצדיק את ראשו
that we want Hashem, so to say, to put His head on all of us.
Have a wonderful day.