A Sim suddenly lost his relationships.

I never paid much attention to the relation panel of this game. It might have improved over time, but I basically think it is too random or relations suggested by game overruns relations initiated by me. They sort by "latest seen" meaning a strong relation your sim did not contact in a long time will sort far down. To me this makes no sense as I would prefer focusing on my sim's better contacts. Because I play rotationally there might be a calendar year between each time I play a house, and most contacts are making little sense to me, if any at all. Perhaps your sim lost contacts because he waited too long to stay in touch?