There is only one reason.
The speech he gives to Scarlet Witch in Avengers: Age of Ultron is the most inspiring things I've ever heard.
He basically tells her that they're fighting an army of robots and he continues on to say he doesn't have a suit of armour, a mythological hammer, or even a rage that turns him into an untouchable beast.
All he has is a bow and arrow.
Yet, he still wants to go out and keep fighting for the people of the city, to keep them safe, because it's his job.
He ends up inspiring her to keep fighting, saving the lives of thousands and inevitably the entire world.
Even the rest of the Avengers.