It makes you feel good. It also makes others feel good, so, win-win. Indeed, being kind to others gives you a sense of satisfaction that makes you happy, which in turn makes you kind. This phenomenon has been studied by researchers at Harvard and the University of British Columbia. They found that being kind to someone created a "positive feedback loop". For example, if you do something kind for another person (i.e. buy them a cup of coffee) it will increase your mood, making you happy. That happy feeling increases the chances that you will commit another kind act, and so on. Similarly, people experience this phenomenon when achieving a goal. Their achievement activates the reward center of their brain which releases dopamine into their system. The dopamine makes the person happy, increasing their self-esteem and priming them for more achievement. It is also seen in pleasure-seeking behaviors such as sex, drugs, and probably even rock and roll.
How do I experience this phenomenon, you ask? Easy. Begin a hardcore drug regimen, kidding, that would be terrible. Just start smiling, yes, something as simple as smiling can positively affect others. You will get some awkward looks, sure, and sometimes even a scowl, but every so often you will get a genuine smile. I promise the rhyming was unintentional.
There are some other basic things you can do to positively affect those around you. You can look people in the eye, hold open doors, say please and thank you, essentially, just basic manners. Manners are small indicators to other people that you give a shit about them, and everyone likes to feel like someone gives a shit about them. It makes them happy, increasing the chances that they will be kind, creating their own feedback loop, positively affecting those around them.
The manners are just the beginning, a very good place to start, and they will influence the rest of your behaviors, as well as improve your interpersonal relationships. We are all connected in some weird way and we can affect each other in ways we can't even imagine. So, be fucking kind, damnit!