Why kiss the altar?

Why do the priest and deacon kiss the altar? 

 
Before celebrating every mass the priest (and deacon) venerate the altar with a kiss. To some this practice seems rather odd, as altars are material objects of stone or wood and don't seem to warrant any particular reverence.  

Kissing holy and sacred objects has been part of various world religions for thousands of years. The practice comes from cultures where the kiss was viewed as a sign of respect or used as a greeting and was naturally applied to objects that represented the divine. As the liturgy developed, Christians adapted customs from their own culture and gave them new significance. Kissing the altar was quickly attached to the actions of the priest at mass.  

The altar receives its importance from the Holy Sacrifice of the Mass that is celebrated upon it. It has been set apart for this purpose and is consecrated by the bishop with Sacred Chrism it when it is installed in a new church. This is why the altar may be incensed at the beginning of Mass and during the preparation of the gifts.

Symbolically the altar is often said to represent Jesus Christ, the "cornerstone" of the Church (cf. Eph. 2:20). During the history of the liturgy the priest would sometimes kiss the altar before blessing the people, symbolizing how the blessing came from God, not the priest.  

Since the altar represents Christ, and being the place upon which the sacrifice of Mass is offered, it is appropriate for all to bow toward the altar whenever we pass it. When the Blessed Sacrament is upon the altar, one should genuflect.

Additionally, over time relics of saints were inserted into the altar and when the priest kissed the altar, he would be venerating the relics as well.

So while it is true that altars are material objects, they have been set apart for a specific purpose and kissing the altar recognizes its privileged role and relation to the divine sacrifice of Jesus Christ