A guy walks down a beach and comes across thousands of beached, suffocating starfish, and a little girl throwing them one by one back into the ocean. "You know you're barely going to make a dent in all of this," the guy says, because he hates kids or something. "The vast majority of them are still going to die. None of what you are doing matters."
The little girl picks up another starfish and throws it into the ocean, and turns to the guy and says, "It matters to that one."
So, OK, sure. You have questions. Why does that man hate kids so much? Are we sure that throwing the starfish is good for them? Does touching damage them? Does throwing damage them? Are they able to survive once they're thrown into the ocean, or is the girl only adding starfish corpses to the bottom of the sea? I don't know. Ask a marine biologist. Have the marine biologist provide you a better analogy, then come back with that story in your head.
Most corny retellings frame the takeaway as "Even one person can make a difference!" or some generic moral that completely misses the unique beauty of this story, which is not about why you ought to help (we've got quite enough about that), but how you can get the emotional fuel to keep helping. To zoom in on that point, imagine that in this story, the starfish had faces.
Oh God, that's horrifying. I'm so sorry.
Anyway, imagine that a starfish can express joy, and that as it flies safely toward the ocean (where it will land completely undamaged in the correct ecosystem), it expresses the gratitude of a creature that was resigned to certain death a moment ago and suddenly sees before it a long life of freedom. It sails through the air in slow motion, shouting "Wheeeee!" or "Freeeeedommmm!" or your favorite inspirational movie quote. Now imagine the only reason you helped the starfish is because you knew it would thank you.
Are you still making a difference? Or just feeding your ego? My argument is that it's OK to try to do both. In fact, I think you need to do both.