Other then due to Disney thinking that two friends of the opposite sex cannot stay friends and that Simba needed a love interest.
Love is blind, yadda yadda. I know that. 'Heard it a million times and preached it quite a few.
You can argue they liked each other as cubs however even then it seemed abrupt. They meet, they act like BFFs, and they fall in love [s]due to Timon[/s]. Their romance also seemed a bit rushed to me.
I think it's one of those cases where you haven't seen someone in a long time and are attracted to them. Like..Genetic sexual attraction, except they're most likely not related.