Random Thoughts on Relationships

Truthfully, these aren't completely random thoughts. They are somewhat inspired by the life situations of friends, family, and just society in general. But for someone not exactly privy to what is in my head (you can thank God for that) they will certainly appear to be random.

We have all experienced that we have friends that seem to be friends only for a season. It's not like you end up as enemies or hating each other; it's just that you lose contact and closeness and that's okay. 

I'm wondering if it's normal (naturally, not socially) to have mates for a season.
Socially we think of marriage as till death do us part. But with many well adjusted divorced friends, I'm wondering if that's more of a cultural thing and perhaps after sharing common time together for whatever mutually beneficial purpose (raising kids, getting each other through school, careers, travel, etc.), separating and going separate ways is a natural thing. Sometimes people evolve differently or at different rates and may just not be as compatible as they once were. 

Can former mates maintain mutual friendships?
I know that many divorces or break-ups can be bitter affairs making mutual friends choose a side. Others seem to be comfortable in having made a good decision and both parties naturally seem to thrive in their new roles and relationships.

I know this is a weird subject to bring up, maybe even a bit awkward. It certainly isn't meant to cause any speculation about my own marriage. It's just something I've been wondering about as my own thoughts on religion, relationships, and life have been changing.

Any thoughts or wisdom to share?
I'm just wondering.

John