I think the factors that influence population changes/movements are things we have noted:
1. Jobs/economy: People tend to go where there are available jobs that allow them to earn a liveable wage. Obviously, some states have been winners and others losers in this.
2. Demographics: People, especially the young, are drawn to larger cities and coastal areas. While the costs of living may be higher, so too are the wages, amenities and social/entertainment options. There may be more jobs too, but also people want certain recreational/social opportunities that larger metropilitan areas and coastal areas tend to provide.
3. Climate: As Nat noted, people tend to be drawn to more temporate climates and away from colder ones. Certain hardships can be avoided and the options for year-round recreation are increased.
4. Immigration: Since much of the immigration into the U.S. has come from our southern border, it makes sense that those people would tend to congregate closer to their entry point, providing that they can support themselves there. The southwest, and to a lesser extent the southeast, U.S. thus has a higher concentration of persons who have entered from other countries, both legally and illegally. This involves many millions of people that have mostly settled in those areas. Also, those regions would have climates more similar to their countries of origin than would northern states. And, if they are looking for work in agriculture or construction, they would be able to find those jobs in states in the south more so than in the north. For comparison . . I don't have the latest figures, but a few years ago I read that the population of Ohio was 98% comprised of native-US caucasions and blacks. Imagine if our population were similar in make-up to California, where I believe upwards of 20% of the population is now Hispanic. With an influx like that, Ohio would likely be up near the top of the list forp opulation growth(Not that I am in favor of Hispanics moving to Ohio en mass . . and there wouldn't be the jobs here to support that influx anyway).
I believe that "happiness" is influenced by many things which are subject to change over time. Years ago, I visited Denver and was told be the people there that many Californians had left their state and moved to Colorado, to get away from high crime and increases in certain demographic groups there. So, while California has always shown population gains, there is also a certain percentage of Californians (or residents of other states) who see negative changes in their state and leave to what looks to be greener pastures. If problems follow the flow of people into Colorado, perhaps people might start to leave Colorado as well. If the publicized water shortages in parts of the southeast and southwest increase, perhaps there will be a reversal of the exodus from northern states to those areas. Things could look very different 20-30 years from now (not that some of us will be around to care).