The first part* of my answer to the question, "Why do you believe what you do?" has to do with the nature and experience of belief, and why I believe anything at all. It is not an answer limited to Christian belief.
This first reason was stated eloquently and succinctly by St. Augustine in his Confessions:
"You have made us for yourself, O Lord, and our hearts are restless until they rest in you."
I believe that because in my observation and experience, it is true.
We are all seeking relationship with and connection to whatever is beyond ourselves. I don't discriminate among religious traditions in making that argument, and I don't overlook those who are hostile to religious faith or indifferent to it. It is empirically obvious to me that we are all looking to ground ourselves in a sense of being or entity beyond us.
I am deeply engaged with the life of my religious community, a mainline Protestant church, but most of my family and friends are not so inclined, so I have some sense whereof I speak.
When people express tremendous resentment and anger with respect to the concept of God, when people tell me that God has been absent to them for longer than they can remember, when people recall childhood Sunday School teachers and nuns and First Communions with sarcasm or hostility, when people argue that religion stands firmly behind most of the destruction and horror that human beings have wreaked upon one another in this world, when people say that they might be religious but for the fact that "Christians" (feel free to insert the name of another tradition) refuse to honor them as gay or lesbian individuals ~ what I hear are people longing to rest in the presence of God, and angry and hurt and resentful that they cannot find a way to do so.
When people tell me, "I just don't know," or "I just don't have a religious bone in my body" or "The whole religious enterprise is ludricous and of human origin," what I hear is a longing to be wrong, a longing to be in touch with that dimension of human existence that does not depend upon entirely upon the rational or scientific.
I hope I don't sound condescending. I certainly don't mean to. I don't for one minute doubt that wise and thoughtful people can find grounds for disagreement with me. I don't intend to reinterpret for capable people what they are able to analyze and understand for and about themselves. But the question asked of me was why I believe what I do, and part of the answer is the spiritual reality I see in human nature as clearly as I see that we are designed to love and desire one another, to procreate, and to care for one another. I see it as clearly as I see that there are jellyfish in the sea and foxes in the cemetery where I walk. I see it because it is right in front of me.
I do not say any of this in a strident or defensive tone of voice. I say it confidently, but mildly. I am not perturbed by disagreement on this matter. I simply see that we are made with an interior space for relationship to something beyond ourselves.
*(I think there are five parts. I guess we'll find out.)