When you are in a relationship, you are aware that it might end. You might grow apart, find someone else, simply fall out of love. But a friendship isn't a zero-sum game, and as such, you assume that it will last forever, especially an old friendship. You take its permanence for granted, which might be the very thing so dear about it.
Not everything is supposed to become something beautiful and long-lasting. Sometimes people come into your life to show you what is right and what is wrong, to show you who you can be, to teach you to love yourself, to make you feel better for a little while, or to just be someone to walk with at night and spill your life to. Not everyone is going to stay forever, and we still have to keep on going and thank them for what they've given us.
When we fall in love with someone there's a moment when we take a picture of that person, an emotional snapshot, that we carry with us forever. If we're lucky, if we're very, very lucky, the person we fall in love with will always resemble that snapshot.