Why does Beric keep coming back?
R'hollor, the Lord of Light, is a mysterious figure in both the show and George R.R. Martin's A Song of Ice and Fire book series. His words – "the night is dark and full of terrors" – sound fearsome, and his most ardent followers, like Stannis and Selyse Baratheon, wind up fanatical and mad. He is also unpredictable. How was Thoros able to wield a massive power like resurrection when he hardly believed? Melisandre, a priestess of R'hollor, was similarly disenchanted with her Lord before she brought Jon back to life.
It's not yet known exactly what purpose Beric will serve, only that he believes it's something major. In "Beyond the Wall," Jon asks him what's the point of serving a god with no clear intention. "I don't think it's our purpose to understand," he admits. "Except one thing: We're soldiers. We have to know what we're fighting for. I'm not fighting so some man or woman I barely know can sit on a throne of swords."
"So what are you fighting for?" Jon asks.
"Life," he replies. "Death is the enemy. The first enemy and the last."
Beric also tells Jon that their purpose is to defend others who can't defend themselves. That they were both chosen for that reason. Perhaps their skills as gifted warriors were handpicked by R'hollor to serve against the White Walkers. Or perhaps, together, Jon and Beric are prophetically connected.