Chemistry is everything. Why would you spend time with someone beyond initial curiosity? Isn't it what makes you realize after a huge fight that there isn't anyone else you would rather fight with?
For me, chemistry is part of an odd, complicated process of how and why I am attracted to someone. There has to be a spark of some sort, something that makes one want to be around the person more. Not just "I had a great time *awkward shuffling of feet*...", but "I had a great time with her." Or him.
It may very well be part of the beginning jumpstart, but it shouldn't go away - the happiest couples I know still have chemistry: that unspoken spark, the fact that actions on their part trigger a visceral and fun reaction on yours.
One of my favorite childhood memories is creating fun games that all us kids would play.
My parents and their friends were big entertainers (huge parties every weekend). Around 1998-2001 (I was 11-14) they had the longest streak of extended partying with the most friends. Most of the people they hung out with had kids around my age.
I would try to create fun things for us that didn't involve watching TV/movies or stuff like that. I wanted to have visceral fun.
I created games to play in the pool, games to play inside, and games to play running around outside. And most of the time they were hits, so that always made me happy!