Sadly, the nation has a pretty good idea how President Obama will approach Friday's memorial service for the Rev. Clementa Pinckney, one of the nine killed in last week's church shooting in Charleston, South Carolina. This is something he has had to do far too many times.

To watch the president at such memorials is to see a man touched by the lives lost and the families depleted, frustrated by the easy acceptance that such tragedies have become routine, and angered that he can offer little beyond words.

Charleston is the 17th mass-casualty shooting of his presidency, the 17th time that one incident claimed at least three lives, bringing to 149 the death toll from these bursts of gun violence on his watch. It is the 11th time that he has issued a statement in reaction. And Friday will be the seventh time that Obama has spoken at a memorial, trying to comfort the bereaved and make sense out of the handiwork of a killer.

He was there at Fort Hood in Texas to remember the 13 gunned down in November 2009. He was there in Tucson, Arizona after six died in January 2011. He was there in Aurora, Colorado, to talk about the 12 slain in a theater in July 2012. He was there in Newtown, Connecticut, when a man killed 20 children and seven adults in December 2012. He was there at the Navy Yard in Washington after 13 were killed in September 2013. And he was there—again—at Fort Hood when three more were killed in April 2014.

It is believed to be a record number of such services for any modern president—especially when you add in the memorial services not linked to gun violence that he has attended: in West Virginia for 29 killed in a coal mining explosion in 2010; in Joplin, Missouri, after a tornado killed 158 in 2011; in Boston after three were killed in the Boston Marathon bombing in 2013; and in Waco after 14 were killed in an explosion at a West, Texas, fertilizer plant in 2013.

Clearly, it has taken a toll on Obama, letting loose a flood of emotions bouncing from grief to acceptance to sadness to frustration and anger in the comments that he has made in the days since the South Carolina church shooting. That he is keeping count is clear from those statements. That he is deeply and increasingly frustrated also is clear, both from those statements and from the progression of his remarks at the all-too-familiar memorial services.

All modern presidents understand that they have a duty to serve as what presidential scholar Stephen Hess calls the "mourner in chief." But all dread the burden of speaking at such services. Since the dawning of the television age when Dwight Eisenhower was in the White House, some presidents have stumbled in trying to fill that role.

Lyndon Johnson was particularly ill-suited to the challenge, as was painfully clear in his response in 1968 to the assassinations of Martin Luther King Jr. and Robert F. Kennedy. Johnson responded more as legislator in chief than mourner in chief, appointing commissions and sending up legislative proposals.

Another president who struggled in the role was George H.W. Bush. He just found it difficult to get through a memorial speech without losing control of his own emotions. In 1989, he could not get through his speech at a Norfolk, Virginia memorial for the 47 sailors killed in an explosion aboard the USS Iowa. "The Bush family's not very good at that kind of thing, anyway," he told reporters later. "But you've got to show them concern "¦ You know how you do it? Pray for strength."

He added, "I did choke up at the end, but I did feel a kind of inner peace about the thing. I felt I had to represent our country. This one was tough."

Bush's son, George W. Bush, was better able to get through such speeches. But Andrew Card, his longtime chief of staff, said it took a toll on the president. After meeting with the families of victims of 9/11, Card described Bush as spent. "He was physically exhausted, he was mentally exhausted, spiritually exhausted, emotionally exhausted," Card said in a speech later.

Well-suited for the task were Presidents Ronald Reagan and Bill Clinton, both of whom were able to talk of the deceased as fathers, mothers, daughters, sons—members of the American family. They understood, as Reagan said in 1985 at a memorial service for the 248 soldiers killed in a plane crash, that he was "here in the name of the American people." Both Reagan and Clinton managed to mix tears and laughter in easing the grief gripping the nation.

Perhaps the most important message from any president at such a time is one voiced by almost all the presidents. "I assure you, you are not alone," George W. Bush told those who lost loved ones in the World Trade Center. It is a message that Obama has stressed in all of his remarks. In Aurora, he expressed the hope that the families might take solace from "the awareness that not only all of America but much of the world is thinking about them." In Newtown, he said, "I can only hope that it helps for you to know that you're not alone in your grief."

Because his demeanor is famously cool and his style decidedly professorial, there were doubts about how well Obama would be able to fill the mourner-in-chief role. He quieted those doubts with an eloquent speech in Tucson after a gunman killed six and wounded Rep. Gabrielle Giffords in 2011. His remarks were widely hailed even by his critics, and gave comfort to the victims' families.

From the start, he left no doubt that he understood his role. "There is nothing I can say that will fill the sudden hole in your hearts," he said. "But know this: The hopes of a nation are here tonight. We mourn with you for the fallen. We join in your grief."

Then he did what has become an Obama trademark at such events—he talked about each of the victims: naming them, humanizing them, making them more than just numbers or statistics by talking about their lives and hobbies and loves. One attended daily Mass, one had a dog named Tux, one was a champion quilter. And one moved all who watched to tears. That was 9-year-old Christina Taylor Green, the youngest victim; a dancer, a gymnast, a swimmer, and a girl who loved to jump in puddles. There were few dry eyes when the president concluded, "If there are rain puddles in heaven, Christina is jumping in them today."

As the death toll from shootings has risen, so has another element in this president's memorial speeches—a demand that the nation do something about gun violence and stop accepting it as what he lamented in his Navy Yard remarks has become "the new normal."

"It ought to be a shock to us all as a nation and as a people. It ought to obsess us. It ought to lead to some sort of transformation," he said then, pointing to how other nations have reacted. "Yet here in the United States "¦ nothing happens." This, he said passionately, "we can't accept."

Two years and multiple mass shootings later, the president will try again Friday. No one should be surprised if, in addition to naming the victims and expressing the nation's grief, he once again gives vent to some of that same frustration.

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