The context of the question was what is the correct way to say l'shem yichud, and the response was that maybe it isn't such a good idea to say it at all.
Regarding the rationale, someone who knows these things better than I explained it in terms that I think are understandable:
If you have your Microsoft operating system with its factory settings, your computer will work just fine. If you start playing around with the settings, you could customize your computer, or, if you don't know what you are doing, you could mess everything up.
When we daven or do mitzvos "regularly_, there is a built in status-quo put into the berachos by the anshei knesses hagedola. When you decide to direct it on your own, if you don't really understand what you are doing, you will make things much worse.