Why is political democracy accompanied by social democracy?
Democracy comes from the Greek demos, meaning "the people" and kratia, meaning power, or rule. Since Grecian times the concept was therefore one where the people ruled, or more importantly, engaged in self-governance. The concept of self-governance implies that there is little or no superior group that imposes its will upon another; democracy tends away from oligarcy and towards consensus. It did not imply the concept of "majority rule," although in modern times it has sadly come to incorporate that undemocratic principle.
As individuals have the power to engage in self-governance, meaning that each shares and has a stake in the politics (from the Greek polis, meaning "having to do with the city,") the social and political relationships between individuals become the same thing. In other words, where a democracy evolves, the social and political hierarchical structure, where one group or individual has more political power or social standing than another, becomes levelled. Furthermore, democracy has this same levelling effect upon the economy -- where individuals possess self-rule or governance, they are free to engage in any transaction where they perceive a mutual benefit. Economic opportunities are not restricted to a ruling class. This is partially what is meant by the concept of a Free Market; it's also the reason Democracy and Capitalism go hand in hand.
Democracy, then, is the great leveller. allowing individuals to exercise their own governance (politics), relationships with other individuals (social aspects), and economic improvements (business.)
Social and political democracy tend to go together because the attitudes that cause social democracy make people want to have political democracy as well.
To have social democracy, the people of a society must feel like they are all more or less equal to one another. Once people feel that way, they will want political democracy. That is because if you feel you are just as good as everyone else, you will want to have a say in governing yourself -- you won't just want to sit back and let others do it.
Political democracy is based on the idea that we are all equal and should have an equal say in government. This is more likely to happen when people believe they are all equal socially.
You can also say that political democracy contributes to social democracy. That's because when everyone has equal political rights, they are likely to think they are equal socially as well.