The preposition 'for' has several uses: For 1....Duration of time..............We walked for two hours. 2....Distance:.......................I walked for five kilometers. 3....Purpose.........................I bought this jacket for you / meet for lunch 4....In the direction of............She left for New York. 5....In favor of.......................We are for the proposal. 6....Considering...................The boy is clever for his age. Usage number 3 and 5 may explain why we say "think for oneself". In #3 the purpose is to benefit you, e.g. think for oneself = don't depend on someone else to think about something that is important to you - you must do the thinking yourself, by yourself (alone) and for your benefit. e.g. cook for yourself = cook something for you to eat (purpose = to provide something that can be eaten by you) e.g. save your money for yourself = save your money so that the money you save will benefit/help/be available for you to use when you need it. (purpose = to ensure (make sure) that you have enough money to take care of yourself. Hope this answer helps...

The expression "think for oneself" means to make one's own decisions, form opinions independently, and not be overly influenced by others. To "think by oneself" means to be alone while thinking--it has nothing to do with the thoughts themselves. So you might say, "She went to the park to think by herself" (i.e., away from friends and co-workers).

Cause that you must think for oneself = think yourself!!! "By" means other sense -"with help of someone".