Until the middle ages men regarded themselves as following the Good Shepherd, and art consequently did not recognize the individual in particular. Classical humanism, an itellectula movement based upon the texts of Roman and Greek orators and philosophers, was a crucial part of the Italian Renaissance. This philosophical movement was based on the idea that every persons life had value and dignity. Francesco Petrarch , 1304-1374, called the Father of Humanism, Italian Intellectual, Poet, and Humanist, wrote "Love is the crowning grace of humanity, the holiest right of the soul, the golden link which binds us to duty and truth, the redeeming principle that chiefly reconciles the heart of life, and is prophetic of eternal good."
 

The early Renaissance marks the victory of individualism and the uncompromising prominence of the individual. An abundance of sharply outlined characters suddenly appears, robust, clear-cut personalities; lawless nature belonging just as much in the gallery of criminals as in that of great men. Character, individuality, power and energy are the passwords of the Renaissance age.  This new humanity, all these rugged and manly figures which the age had created, had also to appear in painting. In contrast to the former preference for beauty of an angelic and tender type, the problem now was to depict energetic and powerful beings; and to replace shy and feminine, though bearded, men in the pictures of the older masters by angular, harsh determined and daring types. The figures which has formerly hovered like spirits above the earth had now to stand firmly upon their own feet and become part of their earthly home.

The  Early Renaissance was a time of  great intellectual and spiritual awakening.   Florentine artists took leadership in the development of a new style of painting focusing on ideal beauty.  During the 15th century Patronage  shifted from the church to the merchant class  and wealthy patrons of the patrician class, such as the Medici family,  began collecting and commissioning works of art.  such as the  Italian art world.  Humanism was  emerging, and religious devotion, though still an important part of people's lives, was being restructured to accommodate the belief that man can be master his own  fate. 

The standard of beauty was measured by rudimentary and uncompromising representation of individual qualities. This may  best explain all the strange physiognomies witch suddenly made their appearance in art; course men of the people with uncouth, overworked figures; peasants, with bones of bronze and pointed weather beaten features; half starved old beggars with sagging flesh and tottering bodies; neglected fellows with bald heads, stubbly beards, and long muscular arms. In place of the former dainty pose, every line is now sinew. Their firm, energetic attitude reflects the entire sprit of the rugged age. -- Richard Muther, The History of  Painting, Henry and Co., London, 1896

The most profound change came with the introduction of the new technique of painting in oils, developed in Flanders, and widely adopted in Italy and elsewhere.