That's a part of "my" concept of parenting too. A child will be what it is called most often. If you tell them stuff like: "You are not able to do this!", "You are too weak!" or similar things, they one day will believe, that they are too weak or can't do whatever they wanted to do.
But if you say: "Hey, good idea. Let's try!", even though you know, they are too small and won't be able to do this very thing, they wanted to do, they gain a ton of confidence. And when they fail, you just say: "Wow, well done. But we have to work on that, so you even will be able to do it better." They learn, that they have to be persistent, without you ever directly teaching them this value.
I remember, when my oldest son was 12 or 13, he took a lens and a handful of dried grass and sit in the sun for hours to kindle a fire. After 3 hours his mother said: "Tell him, that it won't work!" But I said: "Let him, he will see whether it works or not." Well, at that night we had BBC... on the fire he kindled.

The same with nicknames, they had childhood heroes and used to play them and calling each other by their heroes names, so we just did the same, and called them by their heroes names.