Ever have a friend who seemed to change after he got married?
One of my personal philosophies is that people in general are defined by their values. I generally believe the cliche "actions speak louder than words"; that what somebody does is significantly more indicative of their character than what he says (even more telling is the consistency (or lack thereof) between what one says and does). What one does is generally dictated by his values (though perhaps not always consciously), i.e. what is important to him.
I made the observation that there is a limit to an individual's values. Think of values as different-sized balls in a bucket. Everyone's bucket has a fixed capacity. The size of the ball is proportional to the severity of the value: the bigger the ball, the more important the value. So the bucket can only hold so many balls. To change one's values means to shrink, grow, add or remove balls. The bucket of value balls might even be considered someone's soul or essence.
Naturally, most people have "big balls" in their buckets for basic human needs: food, shelter, rest. Those values that are necessary-for-survival aren't particularly interesting in this discussion.
However, the rest of the balls, and their infinitely different sizes, make up each person's identity. For example, we all know (or at least know of) somebody with passion: someone who relentlessly pursues some goal, with seemingly limitless energy. Famous examples include star athletes like Michael Jordan, and industry captains such as Warren Buffet. (However, passionate people are not always successful, but these folks rarely make the evening news.) The passionate types have a singular "value ball" that is huge relative to us "average" folks.
Some times people come across as hypocritical: they say one thing and do another. As I mentioned above, I believe their actions are truly representative of their values. So why say something different? Two theories: one is that that a person is just that, a hypocrite---think politicians and the stereotypical shady used car salesman. The other theory is that the person actually doesn't know himself well enough. A lot of people spend a lifetime trying to find their passion; and because the biggest value ball can be so hard to find, surely the other, lesser value balls can be equally well-hidden.
This is my explanation for why people sometimes appear to change when they get married. Perhaps you and a buddy played golf every Sunday until he got married. Now he spends Sundays with his wife's family. You could argue that your friend changed---he used to love golf, now he never wants to play! However, consider that, to your friend, golf is actually a smaller value ball than you once perceived. Or, maybe his new spouse made him realize that there is another, larger value ball that he has overlooked all his life. It's even possible that the golf value ball shrunk to make room for a new or increased value ball that is shared with the wife. Assuming that the collection of value balls creates one's personality, this suggest that your friend did in fact change. However, I don't think value balls are capable of rapid change. In this example, to your friend, golf was a sufficiently small value ball that removing it from the bucket (or shrinking it sufficiently) to make room for another value wasn't actually as big of a change as you perceived it to be. A final consideration: value balls may change dramatically in a short period of time, but the change is short-lived. In this case, your friend removes the golf value ball from his bucket. In time, though, he realizes he gave up something more important to him than he initially realized. Eventually he'll re-claim the golf value ball, and adjust all the other values accordingly to make room in the bucket. Thus, on the long-term horizon, the bucket's changes are minimal and gradual.
Consider your behaviors, and the behaviors of everyone you know. Of what values are these actions indicative? What values are expressed most consistently? Is what you say consistent with what you do? Think about the hardest decisions you've made in your life---why were they difficult? Was it because of conflicting values? Have your hard decisions taught you more about yourself?