While Squatter probably has it right about why the Elves of The Hobbit are so different from those of LOTR (spoilsport!
) I would like to think that they aren't entirely different, they just don't speak modern English. Remember that scene after supper in Rivendell where Bilbo recites his poem, much to the applause of the Elves? Whatever we think of their seriousness, Bilbo certainly seems to enjoy living there. For him, it's still the place to be if you want to eat, sleep, read or whatever. It's still a wonderful place to be, the very essence of hospitality and comfort, a break from the troubles of the outside world. The Elves of Lothlorien live a lot closer to the troubles and are, in general, a more serious bunch. But even here, there is a similar description of what it's like to be visiting Lorien, and Sam says that, among other things Galadriel is merry.