WHY
WHY MUST YOU MAKE YOUR CHARACTERS FALL IN LOVE IMMEDIATELY

don't, it just makes the whole thing feel rushed and really boring.
It can be done well but most don't know how to do it right. And even then it's more of a chemistry thing.

Chemistry lesson

A fiery girl with sass + a gloomy and lazy guy = could work, they can go off each others strengths such as a fiery determination and a calm(maybe gloomy) nature and improve each others weaknesses.

An innocent girl who loves nature + A stern boy who also loves nature = most like will work, having something in common with a person is a great way to give characters something to actually talk about. Try this for your characters though it isn't necessary.

A greedy, cold but charming guy + a depressed girl with nothing to offer= probabily won't work, if a character doesn't have anything interesting about them to a character who wouldn't notice them with out then it probably won't work.

The lesson wasn't really necessary but this will go on to chapter six.