Friends are for life. And when a friend's life ends, it doesn't mean you give up on all friendship, it's like Starlight Glimmer giving up on it.

You should never forget your friends, but in answer; You are the Princess of Friendship, when your friends have to leave, mourn and never forget. But continue because giving up on friendship to mourn is something your friends wouldn't want. Continue on because there are more friends out there to find, always new experiences and ponies to befriend.

She said it herself, in Friendship Through The Ages

"Nothing stays the same for long

But when it changes doesn't mean it's gone

Time will always get away

As it leaves behind another day"

"Things may come and things may go

Some go fast and some go slow

Few things last, that's all I know

But friendship carries on through the ages"