OK, I've written a slightly unnatural Japanese just now.

I wrote, "なぜ私が diao-chan が好きなのでしょうか？"

We can't say it is wrong, but it's slightly un-natural.

The perfect Japanese sentence is,

なぜ私は diao-chan が好きなのでしょうか？

You know, the particle は is a topic marker.

If the sentence is like that as in the top, it wants a bigger "topic" than "why I like diao-chan".
Like, "私が diao-chan が好きな理由は..."

How to correctly use は and が is one of the toughest topics for the learners of Japanese.