Answer by Balaji Viswanathan:
Historical Right & Left
Given that historically most of us were right handed, right side typically
had positive connotations, while the left side was given negative connotations.
Tom was right in his opinion.
Mary felt left out.
Right got the righteousness & left got only leftovers.
These connotations are also present in the religions that survived the
modern world. Thus, for instance, in Buddhism the path to Paradise is
bifurcated, but only the right-hand side leads to Nirvana. In Christianity, the Son is at the right-hand side of the Father and, in the Last Supper, the favourite of the Lord, the apostle John, is seated at His right, not at His left. 
If you observe closely, you can see this bias everywhere.
In the Middle Ages, Europe had a social order that was analogous to the Indian caste system.
First Estate: Priests and clergymen
Second Estate: Nobility and warriors
Third Estate: Workers
The 3 estates/castes of Medieval Europe.
A lot of the terms that we use in modern democracies came from the French revolution. In 1789, Louis XVI convened a national assembly
of the 3 estates. [Since, the press was not involved, it was called the "fourth estate" by some writers – to mean unofficial and less biased.]
When the 3 estates started getting seated in the assembly, the third estate members [common people] were made to sit on the left of the King while the first two estates [clergy and nobility] were made to the sit on the right, given the historical connotations – right is superior, left is inferior.
The three estates meeting before the French King
During the French revolution [started just after this first gathering] the third estate who were seated on the left increasingly started the anti-establishment ideas, while the right side was working on protecting the establishment. They eventually threw out the monarch, the religious authorities and the nobles.
Since then, left wing meant throwing the establishment and right wing mean protecting the establishment.
Left is about change; Right is about preservation
Given the history of the French revolution, left became those groups that was about challenging the establishment. Right became those groups that was about preserving the traditions.
Since both religion and business is to do with preserving the existing, both groups often allied together. Rapid change was bad for both business & the church. World over, religious conservatives and business conservatives thus got into an uneasy partnership even if they both despised each other. They both were about status quo. They were also more likely to be patriots/nationalists, because nation is an identity that is at the core of stability. They are much more likely to be vociferous in preserving the symbols – flag, history, Constituition, anthem etc.
Since the left was the side the workers came, it traditionally was much more towards equal rights and about spreading of wealth. Most often, leftists hated both religions and businesses. Thus, they voted for governments that put tight clamps on both – such as Communist China & Soviet Union.
People don't remain on the same side throughout. When he was younger, Stalin fought on the side of the revolutionaries since he wanted to change & get the power. However, as soon as he got the power he became the establishment against which others had to fight. In some sense, almost all leftists transform into a totalitarian establishment. No one wants to give up power, while everyone wants to take power.
Even in benign cases, you can see this. Both Mahatama Gandhi and Steve Jobs were lefties in the sense that they were iconoclasts. But, both built organizations that eventually were the biggest on the land & became the establishment.
Left wing means a group of iconoclasts who are intending to throw the past and looking for a change. It can be in economic or religious or cultural dimensions. In a healthy society, both left and right ideas are needed.