How we cite our quotes:
"You risked your life to rescue me; I owe you for that." […] There was a bond between them now, welded in the brotherhood of battle and tempered by the loyalty Murtagh had shown. (42.37)
Friendship is also something earned, not just given. At first, Eragon's not so sure about Murtagh. Can he trust this stranger? After Murtagh helps them fight off the Urgals, though, he proves his friendship through actions.
[…] he was unsure if he wanted Murtagh to stay. I like him, Eragon confessed to himself, but I'm no longer certain if that's a good thing. (48.21)
It can't be all good times and high fives. When Murtagh lops off the slaver's head, Eragon has a hard time processing it. Although the guy may have deserved it, his friend killed a defenseless man. Eragon is forced to reconsider his allegiance. How much does your friends' character matter to you? Would you still hang out with someone whose actions opposed your own morals or beliefs?
The best thing we can do is identify those in power and befriend them. And quickly, too. (53.101)
Here we get an idea of friendship in a strategic sense. The Varden are an interesting group of people, all fighting against the Empire, yet split by competing factions. Saphira recognizes the importance of making friends with the most powerful among them as a way of protecting their own interests. Is that really friendship, though?