This happened many years ago when I was young, about 5 years old (I'm now 21), but whenever I think about this moment it makes me happy. There's nothing more rewarding than seeing someone genuinely happy.
It was at Easter, there was a big open oval and there were about a hundred kids running around collecting Easter eggs from all over the ground. Most kids had at least 10 Easter eggs collected in their baskets, but I saw one girl who was a year or two younger than I was crying because whenever she ran to an egg, someone else beat her to it and ran off. I walked over to her and handed her an egg. I still remember that moment to this day, 16 years later. She smiled, thanked me and wiped her tears away, and I remember my parents looking happy & proud about what had happened.
Today, I always try to help people with their problems by talking and offering advice, mostly related to mental health problems like depression and anxiety, as it's something I can relate with a great deal. It always makes me feel good to know that their pain/struggles can be relieved even a little bit by talking.