Turn of Phrase

I was driving back from the gym the other day, and somehow my mind wandered to how the colloquial meanings of words change over time; in particular, I thought about the word immigrant. Perhaps it was seeing an Hispanic person drive past me in the other lane, or perhaps it was the burrito I was going to have after I returned home; nonetheless, the thought had entered my mind.
I remember being taught about immigration in grade school and learning about the (highly propagandized) belief that each and every immigrant came to the US in search of the "perfect life," aka the "American Dream," and that it was achievable by working hard and being an upstanding citizen. In particular, we were taught that everyone in the world wants to be like us, (a rather egotistical and tangential point) and that we should be accepting of those who emerged from different cultures.
Coming from a town where approximately 1/3 of students in my high school were Caucasian, with the other 2/3 reserved mostly for East Asian and Indian, with a few African Americans mixed in, I am used to diversity; I never found anyone's culture, regardless of their origins, to be that strange. I still held the innate belief that immigrants should be accepted by all, and I held it to heart.
I assume that you would expect my belief to have changed, and I suppose it has, though not without undue influence from external sources; in other words, because the word "immigration" has been so often coupled with pejorative terms of late, I have noticed that some of my opinions have become less welcoming and amiable. That being said, I still act on my geniality to its fullest extent.
To call someone an "immigrant" in America is now a sign of derogation; where it once simply described someone's status as an American citizen, it now has come to implicitly stand for "intruder," "criminal," "interloper," etc.
Why should a simple word change so drastically?
A few years ago, illegal immigration from the southernmost border of the United States became a hot topic, as it does every so often. Those crossing the border were portrayed as attempting to steal very "precious" American jobs by working for below minimum wage, along with giving birth to children in the US in order to have their kids be American citizens, somehow enabling the child's parents to be retrieved somehow later (though it's ludicrous, seeing as the child has to wait until he/she is 18 to apply for residency for his/her parents, and then the typical waiting time is 10 years). Thus, the "immigrant" was tainted; an Arizona law allowed for police to "randomly check" people who seemed "suspicious"; pundits railed against the lack of security along the border, that there should be a gigantic fence, etc.
The same applies to the word "gay"; where it once meant "happy," it is now a synonym for "bad"; what causes the change in meanings? Is it simply one's perception of a certain group as defined by some rather meaningless characteristic?

I suppose there is no final thought to this post. I guess that's how my summer has been- no sense of finality.

That's all for now,
Das Flüg