I'm not sure whether you can find out on your questions. But I'll post here in cases you don't.
Firstly, you have to choose on Being Romantic or something like this. Try to flirt a sim, pick up line, or tender embrance. Continue to do so. The green line below will show up like being friend.
However, if you are friends of all sims, you must insult, argue, or slap until a person is disliked you. Then, try to be nice once. Only once. You must not be that person's friend. Then start flirt or else what I mentioned above.
Try to keep continue on this type, I mean being romantic or something like this. If there is another occur such as hot smoosh or something like this, you must try. Then for a while, WooHoo will happen ^^