In the chat section of the options there's an option called "flirt", in which you confess your love for the character, their response is related to 2 stats, their obedience, and your charm.
If you already have the dependent stamp you are unable to get the love stamp at all, you will need to just reset.
In order to get the stamp you need to raise their obedience without raising their libido too high, if their libido reaches 20 you will get the dependent stamp, so you want to avoid that, all while making enough money to buy exp potions to increase your charm.
If your stats are good enough you can test it by using the "flirt" command, if they suddenly gain +9999 obedience then the next day you will be given 2 chat options, 1 for love and 1 for dependent, pick the right one and congrats! you have the love stamp, it isn't much different from the dependent stamp but it looks neat
The best stat combo that I've found works is level 30 obedience with 3 levels in charm and a charm potion from the shop