Because they can, it is their money, therefore theirs to do as they want. They are college aged, finally making a fair amount of money (as in $15+ a hour). This probably been discussed before, but I am just dumping my thoughts on this. Like other people when they finally have their money to spend, not parents money with strings attached or irregular income. The income is steady and theirs to do as they wish.
They have so many desires and habits that "true adults" have. Such as eating out most if not all meals. Because cooking is for poor people and restaurant food is better right? They finance a new car or truck because I must have my own car. Not the crappy hand me down or used car of my parents. Then there are all the items and "toys" they buy. New computer, maybe a motorcycle, pets, and self funded trips alone.
This all sounds typical, but it is the idea of a young person realizing" I earned this money, it is mine with no strings attached on what to spend!" is perception/idea that came across to me. For the most part when MMM talks about being spendy it is mostly about it keeping up with the Jones's or the better life for their kids.
In the case of my friends, it is a combination of independence, freedom, and possibly being an adult as opposed to what I said above. I do not know if anyone of this age group, co-workers, have kids, or what other relation can concur with this or see this same reason.
Correct me if this has already been discussed and point me to it. This is just something that came to me after hanging with my friends for two years and seeing the continuing drift of how we handle our personal finances.