He was 14 when he first applied , 14 year old boys aren't necessarily thinking about fame as a whole package.
Doing what you always dreamed of - getting a record deal, singing to crowds of thousands, writing and performing your own songs, making a living out of something you love, getting lots of attention from teenage girls....those are the things that would attract them to go on, and of course they achieved all that, beyond their eildest dreams.
But the cost is the privacy for themselves and their family. Not being able to go out without being recognised or papped. Making all your teenage mistakes in the public eye, having people write all sorts of stuff about you - much of which won't be true, everyone wanting to be your friend but not knowing who to trust, having limited input in how you look, what you say in interviews etc etc ...
My teen DD follows an Instagram page which basically seems to stalk follows Harry Styles.Thry seem to know where he is/what he's doing at any given moment. She's disappointed when there is nothing to report. I can't imagine how stressful it would be to live your life where people are literally sharing your whereabouts every day, even just going to shop, or cycling around London
As an aside, Jason Orange from Take That, chose to walk away from fame, but even on fan pages on SM I've seen people post pictures of him simply walking down the street.