Curious Expedition 2

You have the option to have not only humans, but other races of natives in your party.

"Falling in love" is a random party event where you and another, or two of your companions, declare their undying love for each other. For context, this raises max sanity and gives you bad consequences if you dismiss one of the two love birds from your party.

This achievement requires you to either personally fall for a different race party member, such as a salamander, or witness it between two others. This can happen when you sleep in a village or out in nature. I never had it happen just randomly walking between locations.

For full disclosure, I got this in the beginning of the third year of expeditions. During the second, you have a chance to rescue a salamander prince from a mole people prison at the end of the second year.
This is done by breaking him out of prison in their main village during the expeditions second phase, located in a cave. It requires not only green dice but if I recall blue as well (though I only found it once so far, I will gladly credit anyone who can clarify)
My point being that when he is saved, he swears a life debt, which makes him more likely to have good events towards you, and more likely to not have negative events effect him.

The achievement will pop the moment the two fall in love and you are notified of the change, you don't need to finish the expedition for it to count.

Happy hunting (>*_*)>