I always thought it was telling that in the title of the episode “Lovers Walk” there was no apostrophe. It’s not like they’re following a path in Sunnydale called Lover’s Walk or anything. It was just lovers. Walking.
Who are the lovers? Angel and Buffy? Spike and Drusilla? Buffy and Spike? Spike and Angel? Yes.
When Spike makes his speech about “you’ll never be friends” on the surface he is talking about Angel and Buffy, but unconsciously he’s also talking about himself and Drusilla “you’ll fight and you’ll shag” and (in my own Spangel shipping heart anyway) about himself and Angel.
In a prescient way he also describes himself and Buffy in the future “you’ll hate each other until it makes you quiver but you’ll never be friends.”
"Love isn’t brains" is perfect to describe Spike/Buffy because he tells her repeatedly in later seasons that he knows it’s crazy that he’s in love with her but he just can’t help it.