A sense of relief swept through Steve, and he gave his teammate and friend a grateful smile. "Thank you," he said, and meant it. He was right that the gem wanted to be used. Cap honestly didn't know how long he could hold out. He needed to get the gem away from himself, to mitigate the desire.
It felt right, asking for the Asgardian's help. There was a sense of stability to him that Steve hadn't felt in anyone else. It was strange, being able to sense someone's timeline. River's, on the other hand, was like a big ball of wibbly-wobbly, timey wimey... stuff. Like a ball of yarn that had been attacked by a kitten. Only the Doctor's had felt more complex, and that was simply due to the length of it.
River Song was smiling that privately amused smile of his. "Well," he said. "Now the question remains: keep it on Earth, or take it to Asgard? I could think of benefits for either, if you like."