There isn't one.
Life is meaningless because it ends in death.
Obviously your own beliefs about the world, religious or otherwise, will dictate how you feel about this. But this is what I have learned:
"A man can do nothing better than to eat and drink and find satisfaction in his work." Ecclesiastes 2:24 (NIV)
"I know there is nothing better for men than to be happy and do good while they live. That everyone may eat and drink, and find satisfaction in all his toil--this is the gift of God." Ecclesiastes 3:12-13 (NIV)
...and so on...
This isn't meant to be as depressing as "eat, drink, and be merry, for tomorrow you may die", although in the end that is basically what it boils down to.
The best thing to do is take it as a release: don't bother worrying about why in your life; just live, with all the soul you have. It is its own reason, its own meaning.
To quote a Sproutling from Legend of Mana:
"To live... is to live."