I don’t think he got his power because of it neither did he start asking that because he got the powers.
I assume a lot of people ask themselves this question (including me), but they don’t get powers because of it neither do they start asking this just after receiving powers, since it’s very easy to do it.
It was implied in the anime he asked the question before he got the powers, however, it was also said that teenagers have a probability of developing powers, that will disappear though. If the first affirmation was true, it would be pretty easy to get their powers back, just thinking about it. And a lot of people would have the same power.
In my opinion, asking this question is what triggers the power, like the spell before the magic. Every time he wants to possess someone else, he has to ask himself this question. Probably he constantly asked this during his whole life, but in one moment that happened, and he realized he had powers.