Do friendships have an expiry date ?

I'm trying to type up a report and the sounds of a once familiar song which I last heard several years ago come floating into range:

I step off the train,
I'm walking down your street again,
and past your door,
but you don't live there anymore.

And I miss you
- like the deserts miss the rain.

And I miss you -
like the deserts miss the rain.

What happens inside of us when we come to the realization that a friendship has died ? Do we shrug our shoulders, take a deep breath and quietly express 'That's life' type sentiments ? And in doing so, do we have an implied karmic acceptance that no friendship lasts forever ?....that we are all but transients in this journey called life, with people temporarily sharing a collective conscience and then hopping off when another destination (or duty) calls ?

Sometimes it may be due to circumstances beyond our control (eg. changing countries, physical distance, careers, or just growing up) that a friendship dissipates over time...indeed it is a rare occurrence where a dosti will just fracture and snap instaneously. But what about those past friends who still exist in near physical, social (and virtual) proximity, but for which now exists radio silence ? Why did we both move on to different frequencies ? Some questions have no answers....

Irrespective of how a friendship evaporates, do we ever bother to commemorate that time together, the lessons learnt and the lasting legacies that will pass into the next phases of our life ? I think of past friendships from which I learnt much about myself, and look forward to future new friendships that may transpire.

I wish you all a great journey and many travelling companions....