Well it's true, this type of relationship is fragile. Any type of relationship in that matter, I think it just realistic. Understanding that it can end anytime but still treasure what it is current rather than treasure what is in the future but reflecting the current time.
Many people feel this way about their relationships, regardless of gender or situation. It's a defence mechanism, like not getting your hopes up to avoid disappointment.
It's one of the greatest acts of courage, to hope after being disappointed in the past.
I kinda of didn't like Sakaki, he was still going along with the relationship with the mindset that it'll end someday, so why bother.