All angels had the same mission drilled into their heads from the first moments of their creation.
Guard the humans.
It was their one true purpose in life, a task set for them by God himself, and they began their training immediately to prepare for the day when they would finally become full Guardians.
The training could take hundreds of years, sometimes thousands. No matter how prepared the angels felt to complete their training, they would still have to wait until, at long last, they were each called away by one of their superiors and given their own place among the ranks of the Guardians.
Despite how long it took, the angels were patient, choosing to focus on their studies rather than complain about the wait. They were taught all about the lifestyles, behaviors, emotions, and motivations of humans, and they learned as much as they could in the hopes that the knowledge would speed up the process of their advancement.
However, this never happened. There was no pattern to who was picked next, at least not one that the trainees knew. But a few, seemingly random, angels from every garrison were selected throughout each year to become Guardians, as the rest of their brothers and sisters looked on with admiration and occasional hints of envy.
Castiel had watched for millennia as his siblings were all led away, one by one, to become Guardians. He had waited the longest of all the angels in his garrison, yet somehow he was still the most patient. And whenever his siblings returned to the barracks after their busy days of watching over their humans, he devoured all of their stories with endless curiosity.
Anna, his favorite sister, told him the most stories. She told him all about her charge, a teenage girl named Elizabeth. She could talk for hours about all of Elizabeth's highs and lows, including her excitement at her sixteenth birthday party, her heartbreak at the death of her first pet dog, and the thrill of her first kiss. And she told him how fulfilling it was to help Elizabeth through all these experiences and experience them with her.
These were the times when Castiel was both the happiest and the least content.
He loved hearing about the humans his siblings guarded, and he never uttered a single word of complaint, but he found it harder with every passing day to suppress the growing traces of bitterness he felt at the length of his wait. He wanted nothing more than to have a human of his own to care for and help, and he couldn't understand why he hadn't been chosen yet. He could only assume he was doing something wrong, but he couldn't imagine what. He studied as diligently as he was able to, learning every nuance of human emotion, thought, and culture, until he was so familiar with them that they might as well have been his own. But he knew that it made no difference in the end, and still he waited.
And it was the natural frustration that came from so many years of work with no reward that made the day when Castiel was finally approached by an Archangel the happiest of his life.