1) I quit my first job in HS. Worst decision I ever made, and I still regret it because my boss was a good man who treated me well. Living with yourself, knowing that you quit something, is worse than the experience of going through it and finishing what you signed on to do. I was too lazy, stupid, and dumb to live up to my responsibility. I've never quit anything since.
2) Going to Ranger School and realizing I would have to live up to the tab for LIFE.
3) Seeing my friends and family go into combat and come back wounded or dead. How can I quit when my cousin got blasted by a 500LB VBIED and lost a finger? How can I quit when I lost my Ranger buddy and my college roommate in one year? How can I quit when a guy who lost his leg overseas gets a prosthetic and goes back overseas in combat. No Army course is worth quitting after seeing stuff like that.
4) The humble heroes of my family history. My Dad is a Vietnam vet who recieved the Soldier's Medal for running BACK into a burning ammo building that had just exploded and blew him out of it. Quitting doesn't live up to that. His uncles fought in WWII at places like Cherbourg, Italy, Philippines, Normandy. One uncle was lost in the USS Barbel off the Philippines 6 months before the war ended. My Dad and his uncles never played up their service. They were just happy to do their part. As am I.
5) Seeing others accomplish what I want. Seeing my bros get their berets and tabs kept me motivated to keep on gunning. If they can do it, so can I.
6) The guidance and mentorship of my friends, co workers, and family. Sometimes we can be our own worst critics and doubt ourselves. While we may see the worst in ourselves, they see what we want to become and they stoke the flames that forge the hardened sword of iron that will sally forth and accomplish the goals set.
7) Once you accomplish one tough thing (i.e. Ranger School, first combat rotation), you want more. It's addicted because your true self comes to light. You strip back the fluff of your life and the mask of your life to see your true self. You want to see that person, you want to become one with him, develop him, make him stronger, make him better. It's a special feeling, and you want to experience it again.