Sometimes people talk about the get-on-great-together-or-not factor in a romantic relationship in terms of "chemistry." The term serves to remind us that how people interact has much more to it than just how well you get along. It makes one think of volatile reactions or the coming together to make something utterly different than the sum of the parts--depending on your level of pessimism about romance.
I like the term when describing all types of relationships with other people. Some others and me together are toxic, I just cannot stand how that person relates to others and to the world and that person doesn't understand me either, and misunderstanding can fairly often lead to dislike. Others I have become fast and committed friends with after just one hour's acquaintance.
The notion of "chemistry" makes it seems like no work is involved, that things just happen when you are together. This may be a relatively decent description of that magic and fun time at the beginning of a romantic relationship, but if it is going to last, hard works is at some point going to be involved. No two people are so perfectly aligned or complimentary in inclination, belief, goals, etc, that they won't find something about the other or what they have to give up to stay in the relationship that they must find a way to deal with.
Friendships take work too. You can't expect your friends to do all the event planning and hosting and calling, and if you're the one who always ends up doing all of that all the time with no reciprocation, you can quickly start to wonder what the heck you're doing it for. If you're lucky enough to have friends who invite you along on too many things that you refuse, even for good reason, even the most dedicated of friends will stop calling.
Yet going through the friend motions with people I've got no particular "chemistry" with beyond that they're nice and friends of my friends and we do seem to have a base level of superficial things such as race, class, age, lifestyle, in common, is strangely unsatisfying as well. Even in this situation I can't define "chemistry" precisely except that when I've had it, then even walking to the grocery store together was an amazing time, then I've stayed up all night discussing science, politics and religion, then I've laughed until I cried even when our plans together involved nothing so extraordinarily exciting. People with whom I've had friendship "chemistry" have not necessarily been very much like me, and I've grown from experiencing and understanding very different views of the world.
I've had amazing boss "chemistry" before too, and co-worker "chemistry" (the same as friend, really, except we were also working together, and I can't always work successfully even with people I am good friends with) where the right level of challenge, communication and collaboration have brought enormous success to our collective professional endeavors. A great sign about my current job is that I seem to have excellent work-together potential with my boss. Maybe one day that can become "friend" chemistry too.
I'm definitely missing some of the friend "chemistry" of some of my past grand friendships right now. It's easy enough to try to find friends, at work, in hobby clubs, wherever else adults who are no longer in that bountiful friendship resource that is school go to find their dinner party and night-club comrades. But friend chemistry like that doesn't happen on command, at least for me, because friendship is work but is more, too, than just going through friend motions with whoever you can find who should work. Great friendships for me have always been things that satisfied us because we were friends with each other for who we were, and not just because we were available to fill in the role of "friend." Whether we saw that from the moment we met each other or knew each other for years but only really "clicked" when we had a chance to talk at a party, still, something clicked, and the click cannot be made up and cannot be forced.
*Editing note: I'm sorry the previous version of this was so god-awfully full of errors. I admittedly don't edit my blog entries very closely, and that is bad, wrong, un-professional, etc.