Juliet is more mature than Romeo she proposes to him, not the other way around, and her love for him is her first but they don't fall in love to defy their families. They don't know the beloved is a member of the rival family when they fall. Love tends to be blind. People falling in love tend to be irrational and illogical. The love emotion takes over, thereafter nothing else matters.
Love can be destructive. Love, blind love, can ruin your life. Yes, some kind of love is blind