Is “love” significant?

[Jenny A]

Being controlled by the chemistry in our brains sounds almost alien; as though it were ‘predetermined’ in a sense?

Predestination is the bug bear in the closet whether you are theist or atheist. If it's all chemical and physical, than you can argue it's all predetermined and runs like clock-work. If you suppose an all knowing god instead (or in a addition to natural causes) you get a god who already knows what you are going to do as if you'd already done it. Acting as if you believed in either kind of predestination is dangerous to your mental health.

But. I don't think we really exist in any real sense outside our brains. Take away or replace my heart, eyes, legs, etc. and I'm still me. Do a brain transplant and I'm someone else.

And yes what runs the brain is chemical. And chemicals can change emotions radically. Recreational drug use is testimony to that. Damage to the pituitary gland, the kidneys, and other organs, produces enough chemical change to really change personalities.

That said, the working of the brain is so complex that I can't say it feels like a bunch of chemical reactions. And I'll be damned if I going to behave as if it were.