Carpe diem, Latin for "seize the day," is a fundamentally proactive philosophy.
It's not enough to sit back and daydream about all the pleasures the world has to offer or even to appreciate all the wonderful things in life.
Rather, this approach challenges the reader to pursue desires with passion and urgency. A common theme involves the fleeting nature of time.
Youth is over before you know it, and death is right around the corner. Therefore, the challenge, according to these speakers, is to fill every moment and make it count.
How many times have you put something off, because you could always take care of it tomorrow? What would happen, though, if you died today? It's not a pleasant thought, but it is an important one, particularly to the application of a "seize the day" philosophy.
The message here is that the very concept of tomorrow is an unfounded assumption. There are no guarantees that it will come at all.
The mindset is that we must make the most of today, because it is all we really have.