Strange. I thought you were one of those happy.
My guess: this must relate to a relationship. It is not working out the way you want. usually it is personal life that can make one depress so much to the point of wanting to hit a 18-wheeler. on the bright side, this can be very temporary. There will be tons of other opportunities to explore. My suggestion would be to find someone (close friend) to hang out (date) as a distraction.