It's just something the creator added as implication, not something fully explained but they did give an answer here
Viewing post in Bound By Fate ~ Cursed Soulmates ~ (DEMO) comments
The game seems to have "character stats" for MC so it can decide what route you'll be getting, if it's your third or fourth run then I don't think you can get it again. I remember the creator saying that the "Thanks, can you let me go now?" Option is the best for that but after a few rounds it just gives you ending 4