It's not actually a literal, physical list. It's more of an abstract idea of a list; a Platonic list, if you will. It is to a physical list of things that make me happy with the Platonic ideal of a chair is to a chair; and just as one can measure the chair-ness of an object by how far it deviates from that Platonic ideal, and conclude that a well-worn La-Z-Boy has more chair-ness than, say, a hacksaw blade, or Dick Cheney, or the TV show "Friends," so can one look at an activity and tell how happy it makes me by seeing how closely it resembles things on that Platonic ideal of a list of that which makes me happy.
There are many things that make me happy. Science fiction conventions make me happy; flogging people makes me happy; taking pictures makes me happy; pictures of me flogging people at science fiction conventions make me triply happy.
Root canals, on the other hand, not so much. Dick Cheney, either. And mornings. Mornings do not make me happy.
Being awakened early in the morning never makes me happy. Being awakened early in the morning by one of the principals of the company with which I work, telling me that the Web ecommerce system is down and angry customers are calling the toll-free number to complain that they can't buy the company's products, definitely doesn't make me happy.