The following quote is by a South Asian scholar of the past:
To cast lustful glances is the disease of idiots. You neither give, nor receive. All you do is put your own heart in turmoil. You can stare at a girl forever, but you will never get her. You will only get the person that Allah has decreed for you as halal. Since this is a matter which has already been decided by Allah, that is why this is the disease of idiots
Therefore, let us remember next time we look at a girl (or at a boy if it is a sister looking) that we are indulging in the disease of the idiot.