Discipline helps you to achieve goals, yes. So let's say your goal is to get jacked. You're disciplined in the gym and you do get jacked. So you achieve that goal. That may give you fulfilment and joy, if being jacked is important to you. Sure. Your life may become more positive because you're jacked, chicks may look at you more, sure.
But those are fleeting accomplishments, they won't make you happy long term. I mean do you think Arnold, after winning 8 Mr Olympias sat back and said "That's it, I'm perfectly happy now". He was jacked, he got his titles, women were lusting after him.
After a while though that fades and loses importance, he started doing other things, movies, politics...he wasn't happy long term because he achieved one goal.