Makes or breaks most relationships. And being in a band is a relationship. If you don't at least have respect for each other's musical tastes or your senses of rhythms clash or your music theory skills are on completely different levels or you have different ideas about stage presence or image or interacting with fans or representing your self online or dealing with promoters or being professional during rehearsals and shows... then it's going to go downhill fast.
Luckily, I'm not in that situation. And it leads to so many possibilities. Writing songs on the fly. Trying out new creative musical concepts. Making up songs on the spot because you know the other musician is right there with you for the transitions. Giving constructive criticism and not having to worry about anyone taking it personally.
Chemistry is priceless. And it takes a long time to find the right combination of personalities to make it work.
Oh Girl show tonight. And hopefully some video footage of chemistry at work will be posted on YouTube by tomorrow.