A partisan outlook is a necessity for the methodology behind party identification and this process typically begins long before you can actually vote. The social perspectives that drive you to choose one candidate over another as an adult are significantly shaped by the process of socialization that begins in childhood. The family unit is obviously the primary institution involved, but as you grow older you have also been shaped by friends and the media. These viewpoints tend to be reinforced rather than challenged by social groupings; in fact, the social groups you seek out tend to be shaped by the route toward which socialization sets you.
Back to the difference between party membership and party identification. The increase in the number of independent voters over the last few decades would tend to suggest that party identification no longer plays quite as large a role in elections in America. In reality, even those not registered tend toward voting based on party identification. The idea of a true independent not shaped by the social perspectives and policies associated with the two major parties is rarely realized. These independent voters have, after all, gone through the same processes of socialization. As a result, certain ideological precepts are cemented over the time which naturally lead to these people adopting a system of thought that is best associated with either a Republican or Democratic perspective. What this means is that over the course of a lifetime, even an independent who has never been a member of either major party will tend to vote along party lines. The primary difference will be that these people are more likely to cross over a party line and vote for the candidate, but only if stance on the issues of that candidate are more closely aligned to the independent voter’s own perspective. Usually this divergence from party identification occurs based either on personal issues related to the candidate or the voter, or if the candidate is a non-traditional representative of his party. (Think Ron Paul and Joe Lieberman.)
The real problem inherent in the independent faction is that there does actually exist a segment of the population that is actually independent and can be counted upon to disrupt the pattern of voting in line with party identification. These are voters whose history can appear disjointed; voting regularly for Republicans and Democrats, as well as third party candidates. The unfortunate aspect of this potentially revolutionary voting bloc is that these truly independent voters generally have even lower turnout figures than the country at large.