Because life is a fuckin' bitch, my dude. But as others have said, exposure. Throw yourself into difficult situations until they aren't difficult any more. I'm 37 and still stutter, but I caught myself leading a huge presentation the other day for work. If I told my 20 year old self I would be doing that he wouldn't believe me. Take a moment to celebrate the wins. Don't be too hard on yourself when you do have a bad stutter or find yourself embarrassed. Life is too short.