I love the friendship of Jonathan and David.
It says a lot about what true friendship entails; that Jonathan loved David "as his own soul" (Sam 18:1) and that the Lord was "between" the two (Sam. 20:23). I think in the same way that having God in a marriage makes a couple's love and commitment stronger -- having God in a friendship makes the connection more pure and powerful. God always improves everything He touches.
I've definitely seen that in friendships of mine that incorporate the gospel. Whether it's sharing experiences, discussing doctrine, or simply opening up hearts to God's love -- it has blessed me and these friendships immensely. I'm very grateful for that.