By our very nature, human beings exist as physical beings, with bodies that are tied to a particular time and a particular place.
God is completely other. He is "present everywhere, filling all things", as we sing regularly in the prayer "O heavenly King".
So how can human beings approach God? How can we, who are bodily, reach out and touch the God Who is spirit? How can we, who exist in time, become one with the Eternal One? How can we, who die, join in the life of the Immortal?
On our own, we can do none of these things. However, in his infinite love for us, God became human - the Word became flesh.
He entered and became a part of this physical world, and sanctified it by being united with it. He did this to reach out to us, to bridge the chasm between us and God that we were powerless to bridge ourselves.
Now, through the physical elements of his creation, we, who are matter, can reach out and touch God. His energies are available to us through the visible, physical, tangible elements of creation which are accessible to us.
These means of becoming one with God are what we call Sacraments, or Mysteries, for they are indeed beyond human comprehension. Yet, although we may never fully understand with our minds how God does this, we know by faith in our hearts that He does.
Water, human touch, the oil of the olive, simple bread and wine - these are the means by which God exposes us to and infuses us with his divine energies - his grace - that we might grow into his divine life. So we allow ourselves to be immersed in the waters of baptism, we are anointed with the oil of gladness, and we receive within our bodies his own Body and Blood under the forms of bread and wine.
In some traditions, there have been attempts to place an exact number on the Holy Sacraments, and arguments have arisen over exactly how many there are. However, this is not the traditional approach in the Orthodox Church, where we are simply grateful that the God Who saw us afar off reaches out to us and draws us to Himself through means that are accessible to us, for He is indeed our God and the Lover of humankind.