Just the sheer fact that we're asking a "why" question that we cannot reasonably answer is one of the reasons I believe in the Judeo-Christian God, and, ergo, Christianity. The fact that we can't answer the question means that we cannot begin to fathom God's reasoning, and I don't want to fathom God's reasoning. If I do, that means that I understand him. If I understand him, that puts him on my level. I would not like to be able to think that the wielder of infinite power has a level of intellectual ability equivalent to mine.
Second, the fact of the resurrection of Jesus Christ. The Bible says that Christ was raised from the dead, and it has been proved an immensely reliable source throughout time. Because of this (and the failure of several theories to disprove the occurrence of such an event) I must believe the Bible entirely. Actually, a formerly atheist Oxford scholar once said that he wished more people would try to disprove the resurrection (which, coincidentally, if you can, you can destroy Christianity) because then there would be many more true Christians.
This is my reasoning for believing as I do, and I attempt to live as closely to the Bible as possible. I fail constantly.
A lot of the complaints about Christians come about the fakers, those who just want to use the Bible as an excuse. Please do not take me for one of those who fake, as I really do attempt to live as closely to the true Scriptures as possible. If I am wrong, I try to change. This is true Christianity, and we will not know the true Christians until the Rapture occurs, if it occurs in our lifetimes, or until we die, and are faced down by the almighty.