What lead to the renaissance?

There were numerous factors that led to Renaissance, such as economic, political, social, ideological, and cultural. Since the Renaissance started in Italy, there is some an explanation for cultural and economical revival that transformed medieval world to modern one.  
Purely economical reason was a decline of agriculture as the main source of revenue for majority. The production shifted to mass quantities, which was in the hand of emerging wealthy urban class. Since Italy at the 15th century was the most urbanized area of Europe, the accumulation of wealth, capital, and production allowed to support and patronize culture and art on large scale. 
Ideologically was the church discredited and people were looking for answer why God had allowed plague, or why after centuries of relative stability, Europe felt int chaos and warfare. Church divided by Schism was unable to give satisfactory answer, and thinkers and educated elite was looking for their own thinking that was outside of approved church doctrine. The inability of church to control entire education, thinking, and later publishing, undermined theocratic structure of the society build on church prestige. 
The cultural revival was also supported by society that was looking for model that was comparable better than medieval art, culture, education, and philosophy. Only better social, cultural, and political model better than anything the medieval civilization knew was Roman antiquity. Rome had left a heritage from building to constitution in scale that was not surpassed by medieval society. The role of Rome as a possible source of revival was the strongest on Italian peninsula, were traces of this civilization survived until 15th century. 
One of the last impulse for the society was a collapse of Byzantine and renewed interest in Greece culture, literature, and philosophy. The geopolitical shift from eastern Mediterranean towards more centrally located northern Italy in feudal Europe, ensured that the idea of Renaissance spread from there to the rest of the continent.