Depression

Read some depressing and philosophical posts today ... notably this, this, this and this.
These and an unrelated event that took place in my hostel last night inspired me to write this.

This is he last semester that i spent in IIT ... and as every IITian knows, the last sem has the least workload. So i have found a lot of time to just lie down on my bed, stare at the ceiling and think. What were the thoughts that went through my mind during those hours ? Now when i look back, i can't remember feeling very happy about anything.
Although i am speaking only for myself, i take the liberty to say that quiet a few of my friends feel the same.
I can't remember when was the last time i felt really happy and elated, and the feeling lasted.
For example ... consider the case of my IIMA admit. It was something that i had worked very hard to achieve ... it was a dream that i had nurtured within myself for a very long time. I had imagined that moment of triumph. As expected, i was happy ... but that heady feeling lasted for only about a couple of days. Soon my mind went back to thinking about stupid things.
Is there any answer to why this happens ?
Possibly a deep feeling of insecurity regarding an uncertain future ... soon we will stepping into the real world ... away from the security that we have been used to all our lives. Maybe a subconscious feeling that these days of enjoyment can never return again and that life henceforth can only be worse ... with more worries and tensions plaguing us.
This sem i have been happier than usual ... but the intensity of depression has also gone up.
I have come a long way in the last 4 years in IIT ... but as i leave, i wonder where on the way did i leave behind my erstwhile jolly nature.