Boy, howdy, have you misinterpreted the Buddha. I believe most would agree that in context, Buddha was saying, "Life is suffering because $REASONS. But it doesn't have to be, because $OTHER_REASONS." A vast oversimplification, yes, but I feel that it is a much more accurate representation of the quote.
Another famous quote that everybody gets wrong because they cut it short:
"Distance makes the heart grow fonder"
That quote means separation is good for couples, but almost nobody knows the actual quote:
"Distance makes the heart grow fonder for those nearer"
This is saying distance destroys relationships and you will look for affection from someone who is available.
Indeed, if anything it seems like in general the opposite would be true, if common culture adapts a quote to more accurately fit their reality (irrespective of whether this particular quote is "true" or not).
(Also, I can't find any source for your claim, despite plenty of articles on the source of the phrase, such as https://quotes.yourdictionary.com/articles/who-said-absence-..., so I'm guessing that's a myth.)
I didn't mean it as a gotcha, just that people will mangle quotes to support whatever idea they're trying to argue. Ironically it seems I have accidentally done the same thing.
He did say that we turn much of life into suffering by our unwise reaction to it.
"Life is suffering" -Buddha
"Tedium is the worst pain" -Grendel
I realize that first quote is meant to be encouraging, but you can take it at face value to mean what it is: most of life sucks.
Pessimism and optimism are choices we make on how to interpret reality based on our experiences. This might sound like some feel-good comment that you can easily just see things better if you try to frame it differently but it's not. I am profoundly cynical and genuinely believe the world would be a better place if more people would replace their unfounded optimism with skepticism. Sometimes there really isn't a reason to be optimistic other than a desire to feel better about the situation.