I love paying it forward for the simple reason it makes ME feel happy.
It's a twofer. Good deed for someone and I feel happy doing it.
As good as a donut but no calories
When I was a kid, I never understood anonymous donations. Like a typical kid, I wanted credit when I did something good! But as an adult, I do those things all the time and the "credit" isn't the important part anymore.
My mother pays it forward often, and the other day, someone actually did the same for her -- it really made her day. She was getting the $5.99 lunch special at a Chinese restaurant with my dad. My dad is blind and has other medical issues, so he needs lots of help with getting the food into a bowl where he can scoop it up, etc.
My mom noticed that the only other patron in the restaurant was looking at them a lot, and when she went to pay at the end of the meal, it turns out the other lady had paid, saying she wanted to do something nice for my mom for all of the work it takes to help my dad.
Just hearing that someone noticed and appreciated her efforts made her day even more than the cost of the meal, I think (though that part was appreciated, too)!