Thomas Jefferson never explained his use of the phrase "pursuit of happiness" in the Declaration of Independence. He was almost certainly influenced by George Mason's Virginia Declaration of Rights (adopted June 12, 1776), which referred to "the enjoyment of life and liberty, with the means of acquiring and possessing property, and pursuing and obtaining happiness and safety."1
Jefferson's rough draft of the Declaration of Independence used the expression, "... life, & liberty, & the pursuit of happiness."2 In the final version, Jefferson altered the wording slightly to read "... Life, Liberty and the pursuit of Happiness."3
Ellis, Joseph J. "The Spring of '76: Texts and Contexts." In What Did the Declaration Declare?, edited by Joseph J. Ellis, 79-94. Boston: Bedford/St. Martin's, 1999. See especially pp. 88-90.