I do believe that this was a Buddha quote. On the surface, it is pretty obvious. If we have angry thoughts then, of course, we will be angry people. But this, if practiced enough, will define what we are down to the core.
If you can keep your mind pure with thoughts then people, like-minded people will be more apt to approach you rather than if you have a scowl on your face. It’s second-nature, we determine who we are just by our thoughts and actions and everyone you come across will believe that as well. Joy will follow you and you will feel that you are born under a lucky star if your attitude matches and it becomes your core being then joy WILL follow you like a shadow that never leaves.
As an exercise try to catch yourself when you start having the wrong thoughts and tell yourself do you want to be like the person who consistently has this thought? Then go on with your day and try to catch the next negative thought that you don’t want to be defined as.