It seems so ambiguous. I've heard it referenced for education, entrepreneurship, and other things.
John Locke originally wrote “life, liberty and property” — if something like this is written in the Constitution/DOI, it legally reads that as an American citizen, your government owes you land. Jefferson used Locke’s manifest as a lot of the basis for the DOI, but he nor any of the founding fathers thought that promising land or assuring land to every single person in the colonies was feasible or fiscally achievable.
It’s an easy change, reinstates the individual over the government/monarchy, and is probably the first patriotic address that really drives home the ‘American dream’ that has been sought for the better half of three centuries. Not only does the American government give you the opportunity to live (life) freely (liberty), within the confines of the law you can do, buy, own, etc. whatever helps you reach happiness (pursuit of happiness). If anything, it puts power back into the hands of the people... supposedly.
Jefferson also took a lot of his ideas from others. Dr. Samuel Johnson came up with one of the most famous lines in the Declaration of Independence.