What is the meaning of life? It's a very personal question, and one that has prompted a lot of philosophic thought - possibly more than any other question. And there certainly is no shortage of possible answers. I'll go through a number of possible answers, and give my thoughts on why it is or isn't acceptable to me.

• Religious View: Many religions perscribe their followers a meaning of life. Often, that meaning is to achieve spiritual enlightenment, maintain faith, or otherwise follow some deity's creed. Of course, this answer really only works if you are religious, which I am not (explained on the Religion page).
• Platonic View: Plato viewed the meaning of life as finding the highest knowledge, the Form of Good, linking back to his Theory of Forms. This doesn't really work if you believe, like I do, that the Theory of Forms is unfalsifiable and lacks evidence.
• Aristotelian View: Aristotle viewed the meaning of life as achieving eudaemonia, often translated as "flourishing." It is similar to happiness, but it focusing on "higher persuits" like knowledge rather than hedonism. In modern terms, eudaemonia would essentially be the utility function of humans. The problem is that eudaemonia is rather ill-defined; many people will have different ideas about what it means to "flourish."
• Moral View: Perhaps the meaning of life is simply to do what is moral, per some definition of "moral" (as discussed on the Morality page). But, much to our chagrin, humans have not yet been able to agree on a universal moral code. Shocking as this may be, it presents a problem with morality as the meaning of life - namely, we would have to decide on a morality to use as a foundation, to say nothing of those that don't particularly care about being moral.
• Existential Nihilism: In my opinion, the best way to resolve this is to follow in the footsteps of Nietzsche: there is no answer. Life does not have an objective meaning, but rather is inherently meaningless. However, that does not mean you should be eternally depressed. Sartre posits that this meaninglessness can be a cause for freedom - if there is no objective meaning, then we have the freedom to determine our own meaning in life. One person may pursue truth, while another may pursue hedonism, and another still may prefer religious adherence.

I find the freedom granted by existentialism to be quite liberating, as it means I can choose my own path rather than having it perscribed by some external force. It's also a little harder to be judgemental of others' choices when you consider that they may have completely different goals than you; of course, there will still be people whose chosen meanings will be things I, and others, look down on, but that's the price of freedom.