People about Cosplay...
Let me make it simple for you.
These are some factors which make this fun-filled activity very fascinated.
1) It is Fun
We are all fed up of real life sometimes, the animated world always considered as utopia and we like to experience that Utopian life as the way it is – funny and playful. It is entertaining and enjoyable.
You can express your love for the character which fascinates you. This is the best way to enjoy the fandom of a particular character that you really like it and want to hide your personality under the charm of whatever character.
People like to be a center of attention. Getting compliment on your creativity or catching the eyes of your friends towards famous movie character is always flattering.
We all like creativity, sometimes costumes are crafted by hand. The process includes sewing, cutting, using something completely different thing for entirely different purposes make you proud of cracking the unique idea that nobody ever think of.
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.
The only place it should really matter is in competition. Don’t enter a workmanship based contest with something that you bought or commissioned, and don’t take credit for the work of others.
That said, some contests have a category for commissioned or purchased costumes, you would be fine if you entered there.