I suppose writers want to give their treasured pairing the best life/ending and society says having a life partner, children and a white picket fence is the ultimate achievement and society says anyone who doesn't have that is sad(both meanings of the word). I know I'm guilty of doing that for characters without thinking. I did deliberately write a fic where my two characters met up again and they were both single and they had no intentions of being more than friends and that was OK.

It's good to have variety in fic so keep writing:)