People come and go, those who leave take a part of you, but you also took a part of them, and there's always new people adding more and more of them to you, or exchanging, so we're never truly empty.
Things don't last forever and the world is in constant movement, that's what's so beautiful about living. The unpredictability. You just have to enjoy the ride.
Besides, sadness and happiness go hand to hand, they are measurable by contrast, sadness is a sign that happiness existed and you can't feel happiness without knowing sadness. You can't lose something without having it first. I'd rather have it even if it means losing it at some point than not having it ever, wouldn't you agree?