1.
John Adams, from Abigail Adams (pictured). Abigail Adams wrote this to her husband in a December 23, 1782 letter. The original letter can be read here, at the website of the Massachusetts Historical Society.
2.
Woodrow Wilson, to Edith Bolling Galt (pictured). This is from a letter that widower President Wilson wrote from the White House on September 19, 1915, to Edith Bolling Galt, whom he would marry. The full text of the letter is in volume 34 of The Papers of Woodrow Wilson, ed. Arthur Stanley Link (Princeton, NJ: Princeton University Press, 1994), page 491.
3.
Ronald Reagan, to Nancy Reagan (pictured). This is from a letter written on White House stationery by Ronald Reagan, to his wife Nancy on March 4, 1981. From Nancy Reagan, I Love You, Ronnie: The Letters of Ronald Reagan to Nancy Reagan (New York: Random House, 2002).
4.
Theodore Roosevelt, about Alice Lee (pictured). This is from Roosevelt's diary entry of February 13, 1880. On that evening, he became engaged to Alice Lee, whom he married. On Valentine's Day, 1884, she died while giving birth to their daughter Alice. Roosevelt's mother died the same day. His diary entry for that day is simply a large black X with the words, "The light has gone out of my life." The Library of Congress has made scans of the original diary pages, available online here.