Another reason is why you want to choose the seed words yourself? Mostly the answer will be to remember it, but you will fail to remember 12 words. You can memorize up to 6 words. Some may reach 10 words, but 12 words are a lot, and failure in one word or its arrangement will be very costly.
This is if we ignore the possibility of memory loss due to accidents or trauma.
The compromise solution is to generate seed words and try to link them to something you remember, such as trying to make them into the lyrics of a song or poem, or encoding them and hiding them in several places instead of choosing the seeds yourself.
easy way write it down.
What would happen in cases where the subject matter loses their ability to remember those words despite it being linked to something they can remember (eg losing their memories altogether? That's a very poor source of entropy and anybody who knows them very well can easily guess the word combination and gets wallet access.
Possible yes, but highly discouraged because entropy has to be generated securely.