Welcome To The Hellmouth
"Life is short."
Buffy always thought of her life as even shorter than others. It kind of came with being the Slayer. Being a Chosen One always sounds really nice, but in the long run, it's not exactly the best job choice when it comes to life expectancy.
So yeah. "Life is short" has always been her philosophy. One day, everyone dies, and you'll never get those days back. So seize the moment, live for the moment, make your life awesome while you still can.
When she told Willow to seize the moment, she had been telling the truth. She really did want Willow, shy, innocent Willow, marvelous, intelligent, brilliant Willow to do that, to make her life joyous. Willow's life, at the very least, was not going to be as short as hers'.
But that night, when Willow got assaulted, when Jessie got kidnapped, she finally knew what a short life really was. The people who got killed by the vamps, turned by the vamps…those were the people with real short lives. But those same people didn't have "A Calling." Those people were just normal humans, living their lives out, completely naïve of the world all around them. They didn't deserve to have such short lives. There was no point to it.
Buffy had been like them, once. But then she had been called.
So now it was her job to make their lives longer. To fight the forces of evil, as Giles would say. To kill off the demons that plagued them, to make sure the people who made the world spin didn't die and stop its spinning. To save them, protect them, make their lives longer and more peaceful.
And if Buffy Summers died doing this, she would make damn sure that she died with a smile on her face.