I've actually said that to people. This is a true story. Someone I know got a 2340 on the SAT. That's good enough for any college! But this person called me, crying, saying "all me self esteem is gone" and "all my happiness depends on my SAT score". And then "I don't want to live anymore." What do you say to such a person besides shut up and be happy?

+1 temporary happiness! :) Or should that be +2... nom nom nom! It is amusing how anyone can think for a minute that the advice of "Just be happier" could ever actually be helpful... That said, there are rare exceptions where some combination of perhaps the utter stupidity of the statement and the situation can make one re-evaluate their feelings at the present to take a brief break from negativity. But it's very, very rare.