I self-answered my question How to make this hypothesis in a negative form?
Now I want to accept it, but it says that I should wait for 2 more days before I can. Why can't I just accept my answer on my question now? It already solved my problem.
Users are less inclined to write an answer for a question which already has an accepted answer. Allowing immediate acceptance of self-answers would decrease the chance of getting a better answer, which does not only benefit you but also future visitors with the same question. For some questions it might not matter, but the system is not capable of automatically distinguishing those cases. Therefore, you have to wait 48 hours after posting the question.
For more details, see this answer on Meta Stack Exchange.