let (one's) heart rule (one's) head
To do something based on one's own personal desires rather than for pragmatic or practical reasons. I couldn't believe it when I heard he'd bought a sports car, but then he has always been one to let his heart rule his head. I think she knows that a relationship with him will eventually end in tears, but she's letting her heart rule her head.
let your ˌheart rule your ˈhead
act according to what you feel rather than what you think is sensible: Don’t let your heart rule your head. I know you like him but are you sure you can really trust him?