I agree that the definition of love that hooks likes is deficient because of that "spiritual growth" phrase. It's limiting, and alienates atheists like you. Yet the sense of it doesn't need to be religious for the definition to hold true.

When we truly love someone, we support their growth and development. I think the point of adding the word "spiritual" is to differentiate that development from the body. We aren't talking about weightlifting. Also, from the mind. It's not that I want my beloved to learn algebra. Instead, the meaning of the word "growth" here has to do with personality, so maybe "personal growth" would have been a better phrase, but possibly just wasn't popular at the time.

hooks defines that growth specifically as becoming more fully ourselves and connecting more completely with the world around us. That's what we want for ourselves and others whom we treat with genuine love.