There is nothing meaningful or worthwhile in life, just living until you aren't. A lot of comments talk about flow or busyness and there is some truth that we find some happiness in adding to ourselves. That is, in perfecting a skill so that it becomes "second nature." I'm not entirely convinced; it seems like it's just a way to distract your awareness from the reality of your existence.
There is an old Chinese proverb (the beauty of which is probably lost in the butchery of my translation):
"Life is like honey-gathering bees,
After collecting all the honey from myriad flowers,
They age and their labor leaves them with nothing."