As a Gen Xer who feels more mentally aligned with the Millennials than any other group, I'd like to give some unscientific thoughts to this:

1) The generation before the Boomers (the bad nickname "Greatest Generation" etc) were generally very unintelligent, boring people who -- coming out of the agrarian world into the manufacturing world and post-war boom -- thought working a 9-to-5 with lots of overtime was one of the greatest things in the world compared to poverty and wretched work conditions on the farm, and they accrued an amazing lifestyle compared to their ancestors. Culturally conservative, they loved making money, being semi-rich and most were unimaginative enough to love punching cards or whatever they did for 8 hours a day. They had loads of kids simply because they could.
2) The Boomers were not much more intelligent then the old timers (but they were), and they were culturally liberal in their youths (some still are) but became seduced by paychecks and quality of life, becoming some of the biggest spendthrifts and credit lovers in history. Ostentatious consumption under the Boomers reached new heights, and the Boomers -- generally speaking -- were not so vivid or imaginative that most of them do not mentally rebel against the idea of an 8-to-5 with lots of overtime punching cards. A lot of them are so bored that even when they retire, they "come back" and persist in the job market because they literally know of no other way to spend their time in retirement. They have swallowed a lot of the propaganda from the old timers and many (not all) believe the BS of being defined by your work and that it has some implicit value. I also believe that the old timers -- who were amazingly cold and stunted emotionally -- severely damaged the Boomers emotional state, so that many of them are incapable of expressing their feelings, accept suffering quietly and look for outlets for their damaged self esteem in material consumption and pride through possession.
3) The Xers inherited this world of bullshit and this generation was the first to be intelligent enough (I believe each generation gets slightly more intelligent than the previous) to reach a breaking point with menial work and slaving away for a life of debt and no free time. The Xers have many interests and get bored easily, and many were the first to reject the world of material consumption. However, as paychecks are finally getting to nice levels with some of the Xers, you are seeing some of them getting seduced into the materially consumptive lifestyle ("keeping up with the Joneses"), further exacerbated by the pernicious effects of social media which they developed no defense mechanisms for in their youth -- it hit when they were adults. Some, however, align more with the Millennials, who:
4) Either graduated with a ton of crushing student debt or didn't go to college (thus being in a tight bind with economic conditions.) Jobs and wages have been stagnant, and there is simply no room because the Boomers Won't Leave and the Xers are scrambling to find any niche they can to get in -- and the Xers have the years of suffering, so they win out on many more senior positions than the Millennials. Millennials don't reject the idea of a suburban home, a commute or long hours necessarily, but the opportunities are simply not there. They also enter the world at the height of corporate profiteering, wage inequality, the end of outsourced (cheap) labor to the developing world. Millennials are the most intelligent generation to come along so far, and deep within them they find utterly repugnant the idea of slaving away doing Stupid Bullshit Forever and finding their self-worth in it, as they have a multitude of other interests. One caveat to this is the get-rich culture of the startups: here you can see the most whorish of the Millennials come forth proudly boasting of their wealth, importance, and travel pursuits, blasting us from multiple social networks with their most inane thoughts as if they matter, and these emotionally hollow people are more akin to Boomers to anyone else -- and, I have suspicions, will be profiteers of the most libertarian fucktardish variety in our society in the next 50 years. The other Millennials though (which is most of them) are highly interesting people, with a good grounding of living a sensible, balanced and imaginative life who are having an extremely hard go right now.