se préparer = to get yourself ready for something préparer = to get something ready / to make something (ex : je prépare le repas)

If you are preparing for an exam you would say: Je me prépare pour un examen. You can also say: Je dois me préparer pour l'examen. or Je dois me préparer pour mon examen.

It is a little bit hard to explain because the difference is small. ''me preparer'' you use it when you prepare yourself for something. ''preparer'' you use for example: Je prépare mes bagages which means I prepare my luggage. I don't know if you understand now.

it's really hard to tell us why french people like to add "by myself", there is no reason, they just say like that...

I was confused too when I started to learn french. For now what i understand is "je me prépare" contains one more information than "je prépare". "je me prépare" means I prepare something by myself. Just like your example, we d better say je me prépare pour l'examen. If you say je prépare pour l'examen, i think this is weird for frensh people even if they can understand you. What I want to say here is that different language has different habit. For the french, people get used to using "je me prépare" because they get used to adding this information: by myself. For these cases, we cant find a very powerful reason to tell us which way to use but we do have a way that we choose. I ve heard one phrase like this: different language produces different culture. I think it is true.