My sims rarely, if ever, go on dates but I have found that they usually roll a wish to propose marriage shortly after the Engaged moodlet wears off although it does come before that for some, particularly Hopeless Romantics or Family Oriented sims.
Many things you choose for your sims will trigger wish chains. Fishing is great for generating wishes that can lead to large valued wishes like "Catch every type of fish" (only needs 20 different fish to complete) and is worth around 7,000 LTH and even more for Anglers. There is also a wish to catch a fish weighing 1,000kg that is worth, from memory, around 10,000 LTH.
Other hobbies like painting, sculpting, gardening will generate a lot of wishes.
Traits will also dictate the kinds of wishes sims roll.
I don't know what you like your sims to do but whatever it is they will roll wishes once they start gaining skill or having performed certain actions.