To most of us, a purring cat seems to be a cat displaying that it is a happy cat. But that might not be true. A purring cat can also be a cat that is scared or threatened by something in its surroundings. It also might be purring because it is trying to vocalize to get your attention for food.
If you approach a cat that is purring, don't assume it is happy to see you. It just might be feeling threatened. Ear and tail position can be telling another story other than how happy he is to see you!