Because it matters what we do.
I used to wonder about this quite a bit, trying to determine if there is a point to anything I do when my existence is but a tiny flash in the scope of the entire Universe. I felt as though I am so small as to be inconsequential and essentially meaningless. In space-time, as it were, we are basically nothing, almost immeasurable.
Then, as I gradually grew up and out of my addict mind and addict self, I suddenly realized how stupid I was. Everything that exists can be broken down into the same stuff. Therefore, our existence is determined not simply by God's spiritual intelligence but by each and every thought, word and action of ours. We are literally shaping our reality, our world, and most importantly, the world we leave behind.
Even less complicated is understanding that everything we do matters on an individual level. Cause and effect is a universal law, and therefore what we do has a direct consequence to us personally, to our lives and to all that occurs in our lives. If I choose to become a selfish drug addict, my life will reflect that. I have created a life of chaos, strife, heartache, illness, fear, depression and failure. Conversely, if I begin to take care of physical and spiritual health, my reality changes accordingly, sometimes in a big way.
And when we merge this individual understanding of cause and effect with a universal understanding, we begin to realize that not only do our actions shape our own lives, but the entire world... and the world we leave in our wake... the world we leave to our children. So yes, it matters what we do. What we do creates our reality and the world we live in. What we do shapes the future for our children. And that's why we bother to do things. That's why we bother to do the right thing.
God, please teach me that what I do matters and creates the world I live in now and the future I leave behind...