In each sentence you wrote, you asked about specific reasons that caused something, so in the first sentences, they all should include "the" before "reasons".
And then, since in the second sentences someone told the other a bout good ways of doing something, it is clearly that they are specific. He told him a bout the good ways, so he knows them now and when he talks a bout it. so you should use "the" before "good ways".
Hope that helped.
P.S, I am not a teacher.
Student or Learner