You are going to die.
No, seriously, it's true. I'm not trying to be morbid, it's just the way it is. What got me to thinking about it is how often the fear of death is used as a motivational device. It's pervasive in our culture. People ask the classic question, "If you only had 24 hours to live, what would you do?" (Answer when you're 14: Find a willing partner and have as much sex as possible.) It's used as a plot device all of the time in books and movies. (You have cancer, you're 50, now stop having sex and see the world.)
Fear of death is actually a useful thing. I mean, it has helped human beings survive by looking for food, avoiding lions in the wild and, yes, motivated us to procreate. Now that we're all civilized and stuff, we don't fear death the way we used to. Wild animals are not commonly encountered.
And if we're being honest, most people choose to engage in faith because they either grew up with it, or they use it to minimize the concern that there might not be anything after death (afterlife, reincarnation, energy transmission, or whatever). Faith is also useful for explaining away death of others, because "shit happens" is too hard to accept. Not knocking faith here, just demonstrating how it helps delay concerns of imminent death.
I think death very subtly stays there in the back of our minds, however, and it often motivates us. Maybe it causes us to strive for some arbitrary metric of success, or obtain major life events like marriage or having a child. It's probably the root of the midlife crisis, that, "Holy crap, what have I done with my life?" moment.
I admit that I'm no different here. From time to time, I ask myself if I'm really making the most of my life, or if I'm squandering it by spending an hour or two on the couch to watch TV. I think, gosh, I could have cancer or get hit by a bus, and what will I have missed out on? What more could I have done? It really gets to be a meaning of life question.
The problem might be that we tend to treat imminent death as a plot device in our lives. Without the threat, and the perception that we've got time, it's just easier to not do anything that adds value to our lives. Then when we start to ask about what adds value, we're back to the meaning of life question. It's exhausting!
Ultimately, for me at least, I hope that if I were to be hit by a bus, I'd be OK in my last moments with what I've done. Being at peace with who you are, influenced by all of the good and bad in your life, is not an easy proposition. I don't think you need to "live every day like it's your last," because honestly a lot of the best moments in life come just by observation (just ask any parent during a baby's first year). I do think, however, that if you feel like you don't do enough to make what you think is a meaningful life, you need to redefine meaningful or stop making excuses.