One of the things that fascinates me is the tendency that all of us have to project the present of our culture onto other cultures and other points in time. This is, perhaps, most obvious when we see a politician claiming that the people of another nation want the very same political structure as the politician's nation, or that some aspect of modern culture (be it gender roles, the modern mode and trappings of marriage, the nature of our current economic system, etc.) is an eternal and fixed part of humanity and not the fluid and transient thing that it actually is. Similarly, many of the counter-narratives that are developed do a similar thing, projecting the struggles and points of contention of the modern U.S. and Europe onto the rest of the world and the past (viewing all past human societies through the lens of Marx's writings, or through modern political feminism, or through the lens of 21st-century fundamentalist Christianity).
It is often less obvious when historians and anthropologists do so, not because we don't engage in many of the same behaviors, but because we are very good at talking as if we are not. We layer our readings of the past with views gained from our own experience in the present, and then fold these within theoretical models (themselves derived at least in part from our experience of the present) and present them as if they were objective readings of another time and place. A common post-modern critique of mid-20th century archaeological models of hunter gatherer behavior is that these models often required the behavior of hunter-gatherers to conform to post-Adam Smith models of capitalism-steeped economics. And the argument was, to a degree valid, though those making it almost immediately began trying to build new models based on other 19th through 20th century social and political ideas thus proving that they really weren't as different from "the positivists" (as they often labelled the more traditional scholars) as they would have liked to believe.
This tendency is understandable. Human behavior is complex, often downright bizarre, and if we are going to make sense of other cultures we have to find some way in which we can enter the mindset of the people of that culture. As a result, we are forced to try to find something that we either have in common with them or else find some way to boil down some aspect of their culture into an understandable model or formula. Beyond that, we are human just as they are/were, and therefore have the same physical and cognitive capabilities, and therefore there should be similarities that we can understand. The problem lies not in finding similarities that allow us to understand others, but in making sure that these similarities are real and not the product of our failure to understand important and fundamental differences between cultures.
There are many ways to separate illusory similarities or explanation from real ones. One of my personal favorites, due of course to my training, is that of hypothesis testing. You suggest a possible similarity or formulaic way to expressing or explaining people's behaviors or culture, and you then begin to determine what testable conditions will result if what you are looking at is actually true. For example, there is a popular body of scholarship in archaeology referred to as Optimal Foraging Theory, and typically applied to hunter-gatherer cultures. It's fairly complex, but boiled down to it's essence, it holds that people will balance the amount of work required to obtain food vs. the amount of nutrition available from those foods, trying to maximize the nutrition (measured in any number of ways: calories, grams of protein, difficult to get vitamins, etc.) while minimizing the amount of work necessary*. By looking at the food remains found in archaeological sites or documenting the foods brought in by groups studied ethnographically, we can evaluate the success of a particular optimal foraging model for predicting and rendering comprehensible an aspect of human behavior.
Optimal foraging models are successful, but only to a point. There are many foods that people pursue that are of little nutritious quality, or may be highly nutritious but are so labor intensive to get that the gain is nullified, and yet people seek to obtain them. So, using what is essentially an economic model to make another culture comprehensible only works to a certain degree. You then have to look into the specifics of that culture top see why the model doesn't work all the way. And when you start looking for this, you will quickly find that you are alternately stuck between trying to find analogies to your own time and/or culture to make sense of behaviors, or else simply confused by what you are seeing and thinking that the people who you are studying are flat-out nuts.
Ultimately, we have to realize two things: 1) we will never understand others as they understand themselves (and vice-versa) and there will therefore always be room to modify and challenge our conclusions about other peoples and other times; 2) all of our understandings of other people will be, to some degree, dependent on our own position as viewers/interpreters. As a result, we should be wary of holding our views of other times and cultures as being anything but tentative and subject to change. But that doesn't mean that these other people are completely alien or incomprehensible to us. Again, we share our biology with them, and much of human behavior and human nature derives from our biology, however much we might like ot cling to romantic notions otherwise. Moreover, when we realize that we have projected ourselves into others, we have been given a reality check and shown a place where we erred, which means that anything and everything doesn't go. If we are open to having our beliefs about others challenged by our observations of them, if we don't simply succumb to the confirmation bias and ignore disconfirming information, we can move closer to the truth of reality, even if we never quite reach it.
A vocal minority of post-modern scholars and their acolytes outside of the academy claim that the fact that we can never have full and true objectivity means that we must reject the notion of objectivity altogether, even as an idealized goal. Robert Solow (as quoted by the anthropologist Clifford Geertz) has said that "that is like saying that as a perfectly aseptic environment is impossible, one might as well conduct surgery in a sewer." It's a good point - just because you can't remove all of your stumbling blocks and biases doesn't mean that you shouldn't get rid of as many as you can, while committing yourself to the process of removing others as you find them.
*This, incidentally, is one of the models often criticized for being based on modern capitalism. Note the similarities between the basic concept underlying the model, and the notion of a market supporting prices based on supply of a commodity.