A trustee is someone who signs checks. Do you sign your own checks? Then you should be the trustee. Do you want someone else to be able to sign checks? That person should be a trustee, too.
I usually suggest that you include provisions in your Life Trust so that, if your thinking goes haywire, someone you trust can replace you as trustee.
But the only situation when I would recommend that a mentally competent person not be their own trustee is if they are too weak or too ill to get to the bank. I had a client who could not get out of bed. I told her, in no uncertain terms, that she was not qualified to be her own trustee. She would have to name someone else. She would still be in charge, but she would have to delegate. She was happy to do so.
For some other kinds of trusts, you should not be your own trustee. But everyone should have one trust, their Life Trust, that is under their direct control. Unless the Settlor (the person whose trust it is) is bed ridden or mentally incompetent, the simplest arrangement is for the Settlor to also be the trustee.