A crush means that you are attracted to someone but whenever you are not with the person, you do not feel anything for that person anymore. It is even likely that you do not think about that person because you just feel happy whenever the person is near. A one-sided love is usually more painful because you are in love with the person.
You will do everything in your power to make the person happy but you know that this is not possible because the person does not feel the same thing that you are feeling. It can be enough to make people feel sad and depressed about their current situation.