I had this happen to one of my Sims who was using a treadmill at a gym. He developed the "Closer from Happy Memories" sentiment toward Naoki Ito from Mt. Komorabi and again with Duane Talla from Sulani.
The first time happened when my Sim was using a treadmill. Naoki got on the treadmill next to my Sim and immediately initiated a conversation. While they were chatting my Sim developed the "Closer from Happy Memories" sentiment toward Naoki. The same thing happened several days later when my Sim was at the same gym using a workout machine. This time it was Duane Talla who came up to him and started chatting. Several minutes later my Sim developed the "Closer from Happy Memories" sentiment toward Duane.
What struck me as odd about this sentiment popping up is that my Sim had never developed the same sentiment toward any of his friends while they were at the gym working out, just the two pre-made Sims that he had barely met.