In theory one could ignore his own exception-making choices, though in reality this leads to cognitive dissonance. Cognitive dissonance leads to suffering. Thus, choices that involve making an exception for oneself lead to suffering. This grounds the categorical imperative in experience.
For example, say you are a European American, and you want illegal immigrants deported from the US on grounds that they are taking away jobs from people who rightfully live in the US. If someone points out that European Americans are here in the first place because their ancestors took away lands from those already occupying the US, you would likely experience pangs of dissonance. You would have trouble justifying that you are "right"; even if eventually you are able to, it's likely you'd go through a fit of defensive thought before becoming comfortable again.
Cognitive dissonance causes suffering in oneself, but also suffering to others. It can make one lash out against those who hold the contradictory view. The existence of people who can bring about dissonance poses a threat, and thus arises a motive to suppress/change/eliminate such people.
Possible challenges to this argument:
- One can ignore facts leading to contradictory thoughts, and thus need not suffer dissonance. I'd argue that the existence of such facts is like a ticking time bomb -- they will eventually be considered, the mind will realize the contradiction, and there will be dissonance.
- The categorical imperative is intended to be The moral principle, but there are other causes of suffering besides cognitive dissonance, especially in terms of harm caused to others. While I think cognitive dissonance is a significant cause of suffering, and fights amongst people, I certainly can't say it's The cause.