I hate this sort of sentiment. You can't tell an upset person to 'be happy!' any more than you can tell a person with a broken leg to carry on walking and stop being in pain, you big wuss.
You can smile and grin and pretend you mean it but if you're upset about something then you can't just suddenly decide 'hey - life's too short! I'll be happy and smile and canter in the meadows!'.
Life is about the bad things as well as the good, in so much as life is about anything at all. Pretending bad things haven't happened does nobody any favours and sometimes you need to let it out to make you feel better. Sometimes you can't help it at all, especially if you're depressed, and sometimes you'd really, really like to be happy but can't. Spam messages aren't going to cheer everybody up, no matter how many miracles they promise. They only make people who feel happy and contented already to go 'ooooh, that's a good message' and forward it.