Seems like the main reason you'd want a new language is that you start discovering patterns in your code that existing languages don't handle well. But there are a bunch of problems with creating your own language. You'll be missing out on all the libraries and frameworks that are built up for existing languages. You'll spend a lot of time designing and implementing the new language, which is all time you don't have to spend on the real programming task. You'll spend a lot of effort convincing other developers that they should use your language. And, you'll have a hard time recruiting and training new developers.

Why not write in a language like Lisp that lets you extend the language as you discover new patterns? Then, you get all the power of a new language with all the benefits of an established language.