Lack of Passion is Fatal.

Chemistry between two people is something that cannot be forced.  It's the natural flow of things between two personalities striving in togetherness; it's the product of a mutual attraction.  Everything is simple and feels normal, right: the careless banter, physical closeness, a certain sense of comfort.  Chemistry is achieving that harmony without actually putting a ton of effort in.

It's what you do with that chemistry, however, that determines where the relationship goes.  It can be great or it can fall devastatingly flat.  After all, chemistry can only be so much of a relationship.  Good friends will always have good chemistry.  Lovers, on the other hand, have something more.

There's a certain passion attached to being in love: a passion to be with the other person, to make that person happy, to make it last.  Passion isn't as naturally-occurring as chemistry, however.  Passion arises from the desire to make the chemistry withstand time, tribulations, distance.  With passion comes effort and with effort the relationship achieves greatness.

I guess it's why relationships between people with amazing chemistry between them don't always work out.

Chemistry doesn't break your heart.  Passion and the lack thereof does.