I'm rereading the Harry Potter books and I came up with a thought, I bet that Sirius cried when he saw James in the after life, not like he was happy, but he was upset. He was sad because he could protect Harry like he promised James. And James comforted him letting him know that he did well.
Sirius(crying): I failed you James, he is out there alone and doesnt have protection!
James: it's okay, you have done so wo much to help him, if it werent for you, harry would have no hope and no happiness in his life you gave him something we couldn't, parenting advise, your time on earth was short but well lived my friend.
*Sirius sniffling*: thanks mate