I need to confirm this in a new game, but something a player did made sense in the context of triggering Denny's 4th Rival Heart Event.
The Rival Heart Events for each couple appeared to be 'tied' to specific activities. With respect to Pierre, it is cooking and recipes. With respect to Sabrina and Vaughn, it is mining. With respect to Denny and Lanna, it is fishing and with Elliot, it is farming.
You need to fish prodigiously in order to trigger the 4th Rival Heart Event between Lanna and Denny but players who have caught many fish slowly have failed to trigger the Event. One player wrote that, when she caught 50 fish finally in a single day, the 4th Rival Heart Event finally triggered.
This may be the key. As previously stated, I need to confirm this in a game of my own, but it is worth publishing here as a decent hypothesis.