Sometimes comes an event in peoples lives that influences them to make a decision that will set them on the path that their life is suppose to be on. Tony Stark had already had such a event happen to him when he had been kidnapped in Afghanistan and escaped. Instead of going back to turning a blind eye and still creating armaments, he made the decision to step up and be a hero as Ironman.
Sometimes a person is a born hero, they just need some help to reach their potential as was the case of Captain America, Steve Rogers when he was given the Super Solider Serum. It was his decision to fly a plane in to Arctic waters that set him on a path to be where he was needed to avert disaster in the future.
Sometimes the event that happens is based on good intentions, but out of their control. Example of this is Bruce Banners accident that unleashed the Hulk when trying to recreate the Super Solider Serum. It wasn't till a new friend suggested that the Hulk wasn't a monster, but a force to be reckoned with, that Bruce started to see that he still had choices in his life.
Sometimes its a series of events that molds a person into the man (or god) that he needs to be. Whether it was learning humility to be worthy of his powers, discovering his arrogance had alienated his brother, or finding he didn't like who he had been in the past, Thor was not afraid to face the challenges in his life. His is a path isn't so much one of becoming a hero, but of becoming a better person to reach his potential as superhero.
Sometimes it's another person making a different call that leads a man to have the guts to do the same when he sees his orders as a waste of potential. Who knew when Coulson drug a dangerous, suicidal marksman into SHEILD that it would lead to the recruitment of the most dangerous assassin in the world and the formation of the elite Strike Force: Delta. Clint Barton and Natasha Romanoff early lives were both places of darkness that they dragged themselves out of with every bit of physical and mental strength they possessed. They are the example of persons changing their life's path by pure force of will.
What decisions will be made in the future?