Three Reasons

Greg Mankiw very succinctly gives three reasons why people who largely agree on "the facts" and are reasonable can fall into either a conservative or liberal mindset.

Arthur M. Okun, who served under President Lyndon B. Johnson as chairman of the Council of Economic Advisers, wrote that the big trade-off faced by society is between equality and efficiency. We can redistribute income to give everyone a more equal slice of the economic pie, but as we do so we blunt work incentives and the economic pie shrinks, he said. From this perspective, the Democrats are the party of more equality, and the Republicans are the party of more efficiency.

Another view is that the important tradeoff is between community and liberty. As members of society, we have goals we want to achieve with others. But as we reach those shared goals, we are asked to sacrifice some personal freedoms. From this perspective, the Democrats are the party that emphasizes communal values, and the Republicans are the party that emphasizes individual liberty.

Finally, there is the issue of how much one trusts centralized governmental power. Democrats tend to want to expand the scope of the federal government to improve the lives of the citizenry, while Republicans are more fearful that centralized power leads to abuse and lack of accountability.

The whole thing is here