I suppose we don't need science to tell us that being nice makes us feel good, but it's nice to know anyway. And here are some more detailed benefits as to what's going on inside you with your kindness:
Physical Health--Kindness can reduce stress, lower blood sugar, and boost the immune system.
Mental Well-Being--Acts of kindness lead to improved mental health, as they can increase happiness and reduce anxiety and depression.
Hormonal Benefits--Kindness can trigger the release of oxytocin, often referred to as the 'cuddle hormone,' which promotes feelings of love and connection.
Mutual Benefits--Kindness not only makes the recipient feel good, but also enhances the well-being of the giver, creating a positive feedback loop. 
I've posted a list of ideas for you to achieve all of these effects. Feel free to write in with your own acts of kindness and I'd be glad to put them on here!
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