An effective thesis statement should be

based on opinion
clear and concise
just a few words long
open to interpretation

In my opinion internal conflicts

Explanation:

The reason for my thinking is because people always battle with themselves. For example; depression. I have really bad depression and it always makes me doubt myself, feel terrible about myself, etc... And to be fair external conflicts can also cause internal conflicts which is another reason. The is not anyone who does not battle with themselves internally and doubt their choices