My goal in life is to create the conditions for world peace in my lifetime, and to do so by making the benefits of meditation accessible to humanity. I tell that to a lot of people. I am amazed how few people actually ask me what I think of my chances of success. Truth be told, I think my likelihood of success is zero, or if I'm very lucky, a very small positive number very close to zero.
I do it knowing my likelihood of success is zero or very close to zero. One reason is this is such an important goal that any non-zero probability is worth pursuing. Another reason is even if I'm only very partially successful, it will help a lot of people. The world population is about 7 billion, if I'm only 1% successful, my work can benefit 70 million people, and if I'm only 1% of 1% successful, that is 700,000 people I can benefit. That is still a lot of people.
I think the real reason I do it is simply because it is what I do. That is it. I am reminded of a Zen story:
There was a military doctor in ancient China who served during war. He was a hardworking doctor and treated many soldiers. But after a while, he made a very disturbing observation. He realized that soldiers he treated would either get injured again, or die in battle. Hence, he began to feel that nothing he did for the soldiers was of any consequence. He started asking himself over and over, "Why am I doing this?".
After a while, he suffered a nervous breakdown, deserted the army and escaped to a Zen monastery where he learned Zen. After many months of meditation, he finally figured out the answer to his question, and happily returned to treating the troops.
Finally, somebody asked him, "So you figured it out? Why are you doing this?". And his answer was, "Because I am a doctor".