Think of your thesis as a statement that remains to be proved. It commits you to showing your reader that it's founded upon good evidence and sound reasoning. That is, you want to show that you know what you're talking about, that you've investigated the matter thoroughly, have considered the implications of your findings, and are offering in your thesis not mere opinion, but a carefully thought-out conclusion. This job of uncovering and displaying your reasoning is the next step in writing a thesis/support essay.