Why do you fast so much? We don't fast all the time - just most of the time!
Prayer and Fasting is something that brings our souls and bodies “under subjection” as St. Paul said (I Cor. 9:27), so that we might be pure and holy. Jesus said that when He had gone, His followers would fast. Like the early Christians, we fast so that we may learn to control our appetite for all things that are not good and holy. It is not about earning salvation, it is a tool to help us work out our salvation with fear and trembling (Phil. 2:12), enabling us to train and strengthen our wills so that they can prepare ourselves to encounter Christ.
Specifically, we fast each Wednesday to commemorate the day when Jesus was betrayed and each Friday to commemorate His death on the cross. In addition, we fast during the entire Lenten period and the entire Advent period, as well as during other times during the year.
Orthodox fasting practice, when followed strictly, means that the believer does not partake of any animal products from vertebrates (i.e., no meat, dairy, eggs, etc.), nor of olive oil nor wine. These choices reflect the desire to do no harm on these days, as well as giving up certain staples of life.
At the same time however, we should never approach fasting with a legalistic mentality. Fasting is for the health of the body and soul so that we can prepare for the salvation of the soul and body. If there are ever any questions about fasting, or if there are issues surrounding someone's health (i.e. if one is elderly, pregnant, or perhaps too young to fast) they should seek the council of the priest so that he may help you decide as to how to exercise discretion when fasting.