The Conundrum that is the Sims

Feelings come from attraction, a hormonal and mostly subconscious thing. We all have a mental picture of an ideal partner.
In practise people can learn to love the "second best" choice. Or to the contrary the on first sight ideal partner expresses their affection in a way your sim doesn't value. Then all their great traits won't help, the relationship just doesn't work. I wouldn't call it a teenage thing, rather a mature one. 

Game-mechanically attraction/feelings come from the traits you picked for your sim to favor in the Likes/Dislikes category: Lifestyle, Character, Clothes type, Hair color, Clothes color.
Romantic satisfaction comes solely from the category "Romance style". A sim who receives a lot of interactions matching their preference (Give gift, flirt, physical closeness or emotional closeness) gains romantic satisfaction. It's thus very easy to make a couple work despite them not being attracted to each other. 

Also keep in mind that attraction can change the more your sim learns about the other, so the "neutral" of your couple isn't set in stone. Give the kiddos a chance!