Reasons might be sad (for no apparent immediate reason)

1. The day before or earlier to that, you might have not got what you wished or not achieved what you desired.
2. You have not been successfull
3. You have been failing for quite some time in meeting the deadlines or following/implementing your time table
4. Somebody spoke to you harshly or hurt your feelings or said something which you did not like that much
5. You are treated like a kid or inexperienced
6. You have been not utilitzing your time properly (an indirect reason)
7. Broke relationship with some good friend (or at least not talking to him/her for quite some time)
8. You are probably trying to escape from some thing and just acting as though you are sad
9. You are playing a prank on us or trying to test the human tendency in responding to questions like this
10. My coffee addiction!