Back in my late high school days, a teacher told me something that has stuck with me to this day.
He didn't necessarily "aim" the comment at me (as I believe it was directed towards one of the more popular girls in the class) but it sank in me like Jason Voorhies in Crystal Lake (Happy Spooktober!)...
He made the statement that the friends we all have now (although, "now" was back then) would most likely not be our friends in the future...hell, even the near future. He claimed that, in high school, you have, let's say, ten friends. Friends that you hang out with regularly...that you socialize with...that you seemingly trust.
As you gain age and maturity, the number of friends decreases from the ten, to around five. The years continue to pass.
That five becomes three. If you're lucky, by the time you're an adult, you have one left from that original ten.

Sure, as one carries out all of life's chores, more friends (and lovers) are made (and lost), replacing the ones that fall to the wayside, confirming the universe's harmony/balance.

If anything, that certain teacher's comment, although somewhat insignificant at the time, makes me believe that change is natural...and inevitable.

"It's evolution, baby".
Rather, social Darwinism.