"If God has a reason for allowing pain and suffering, yet he has the power to change that reason in order to eliminate pain and suffering, then that reason is no longer good, is it?"
No, if God changes the good reason in order to eliminate pain and suffering, he would be changing what is already good. Does He need to do that?
"Then, if pain and suffering is better before going to heaven, why have heaven at all? Just leave it as its is."
Because heaven after pain and suffering is better than no heaven after pain and suffering.
"Isn't that what Jehovah's Witnesses believe? Temporary harm wouldn't be necessary for an all loving, all powerful, all knowing god. What you describe is a less than all loving, and all powerful god."
Well, then that would mean both you and Jehovah's Witnesses aren't logically correct.
"Isn't it arrogant to presume the trials and tribulations of this life are preparing you for something better, especially when there is no objective evidence for making such claim. If there were a god, he/she may not be all loving."
Of course there is objective evidence, including but not limited to the historical testimonies of those who witnessed the life, death, and resurrection of Jesus. The objective evidence of the universe, the earth, life on earth, us humans and the unfathomable machinery that is the human body, and DNA the digital code to build it - all points to design and intelligence, a mind. You observe the same objective evidence, but come to a different conclusion (which involves a lot of denial, and a whole lot more faith, ironically).
The arrogance in your thinking is that you're putting your perspective on the same level as an all-knowing being, which, even if you don't believe in an all-knowing being, is logically absurd. The religious aren't doing that.