Generally speaking, compared to the Middle Ages, there were higher standards of living, greater economic opportunities, and a higher life...

Firstly, if there is a specific context to which you are referring (e.g., a book that you are reading), please be sure to include that so that we may help you with referencing. Otherwise, if you are speaking merely in terms of historical contexts, I would have to disagree with you slightly.

The Middle Ages, sometimes still referred to as The Dark Ages, generally get a bad rap. It is true that those who lived during The Middle Ages were more concerned with religion, but this was due to the desire to firmly establish Christianity in the West, which remained under perceived threat from pagan Vikings in the North -- who frequently sacked the continent -- and Muslims from the East. 

It is also true that literacy did not begin to expand until the Late Middle Ages (c. 1300), as romances began to include the middle-class. Geoffrey Chaucer's The Canterbury Tales was written for and about such an audience. However, even before this time, there was an appreciation for illuminated manuscripts, romances, and visual art. It is just that these pleasures were only accessible to clergymen and members of royal courts. 

Economic opportunities existed during the Late Middle Ages through guilds. However, you are right to say that such opportunities increased during the Renaissance -- particularly since the Renaissance coincided with The Age of Exploration, which expanded trade opportunities for some nations. 

To answer your question, I would say that Humanism was the reason "for the rebirth of the cult of beauty." I left out the first part of your assessment because "the heroic ideal" was actually more preponderant in The Middle Ages. Look at the romances written from 1150-1500 in Middle English: King Horn, Guy of Warwick, and Sir Gawain and the Green Knight. All are romantic heroes. One could even look to Beowulf, written during the Early Middle Ages, for the construction of the heroic ideal. 

Humanism was centered on the belief that rational approaches were key to solving most of the problems of civilization. It also subordinated the Church's absolute authority, arguing that people were not innately sinful, but innately good. Therefore, human ideas should take precedence over those centered on the divine or the supernatural. The best examples the humanists had for this elevation of human ideas and creativity were those from the Classical world. It is no coincidence that Italy was the center of the Renaissance: the remnants of Ancient Rome's greatness were all around.

"The cult of beauty," as you call it, elevated Classical ideas about form and, most importantly, shamelessly depicted the nude human body. The body was not a source of sin, but a source of beauty for Renaissance painters.

In paintings from the Middle Ages, only religious figures were depicted and they were always clothed. Renaissance painters also depicted religious figures, but individualized them and allowed them to emote. However, Renaissance painters also depicted the pagan gods of the ancient world. In some paintings, religious figures were represented in forms that resembled ancient sculptures of pagan gods, allowing for an intermingling of religious and mythical contexts.