People about Cosplay...
Putting aside for the moment that the tone of the question doesn’t imply the most open of minds, cosplay is – simply – acting. Do you consider Mel Gibson, Ken Branagh, David Tennant, Cumberbatch, et. al. immature for wanting to dress up and act as Hamlet? Cosplay is a similar – if not identical – mindset: people, inspired by characters, possessing the right combination of talent, bravery and self-awareness, performing as said character for an audience. Whether it’s a convention stage or a Broadway stage makes no difference. The person who portrays Deadpool or Master Chief is very much spiritual kin to the person who plays King Lear or Hamlet. It’s wrong to praise one and denigrate the other based on the source of their inspiration, for to declare which works are “great” and which are not is subjective or worse. Still, even if most consider it blasphemy to compare Marvel to Shakespeare, the motivation to portray characters from either realm is identical and the healthy mindset WON’T criticize this, realizing that whatever it doesn’t “get” is not automatically inferior or invalid.
PS: Ironically you will find, if you but ask, that most cosplayers as children were very successful Trick or Treaters, as Halloween is quite often the “bug” that bites them for their subsequent motivation.
For me, cosplay is the art of bringing a fictional character to life using yourself as a medium. As people have already mentioned, cosplayers put a lot of money, time, and energy into their craft. Some will go to extremes to lose weight or learn a technique outside their skill level just to try and cosplay that character.
I really love cosplay and I find it is a mirror of how you will treat a lot of things in life, whether that is your job, your relationships, etc. The typical time and project management questions always appear (“Can I finish this in time for this convention?”, “Do I have the resources?”, etc.) but also some that you may not expect (“Should I just stitch this poorly to save time?”, “Will cutting a corner here affect the project later on?”, “Should I invest the time in learning this new technique and apply it to the costume?”). The way you answer these questions are exactly the same way you will answer these questions in real life.
That’s why cosplay is such a great way to make friends than other activities. Not only do you have a great excuse to geek out to like-minded people about your passions, you also can see whether or not they do as they say. You really get to know the people and can truly bond with them.
What is the appeal of acting, and even role playing for that matter? Cosplay is firstly an opportunity to completely put yourself into the character’s shoes, act out a role. Additionally it is a way to showcase your craftsmanship. Just because you view something as juvenile doesn’t make it unappealing. You should be criticizing community theatre if you have that view.
Personally I feel that cosplay gives an opportunity to also put aside your own shortcomings and be someone else for the day. I feel exceedingly confident and friendly in a costume and when I see someone else in a costume, I can “know” who they are and be recognized myself. There’s a level of comradery and confidence in it. Hope that helps.