Sally (or Anderson if you prefer), really liked Sherlock in university. Maybe they went on 'dates', maybe they just attended classes together and did what normal university students do.

Sherlock didn't return their feelings, and they've kind of always begrudged him a bit for it.
Maybe he said he didn't care about them, or just brushed them off.

No villain!Sally or villain!Anderson please. Not necessarily villain!Sherlock either, just perhaps not understanding how to do relationships when he was younger.

Reactions when Sherlock and John reveal themselves as a couple.