Are people who have a crush on someone generally delusional with respect to their crush?
Unrequited love is so common that it is funny. Most childhood crushes begin as such because children are unable to identify who is interested in them. More difficult it becomes when no one is inclined to be in love with them. Being a normal person attachments form to those nearest who show compassion or have some physically appealing trait. Such beings irrational behavior based on the lack of psychological health in a person's life. Therapy can help identify why a person needs love from that individual and why they can't accept the lack of interest. Then it can help identify more suitable candidates for love who bear the same attractable traits. Finally it helps someone develop greater self-esteem and help them move on and cope with the strong pain of rejection. 

Due to the symptoms of their illnesses, those with dependency, attachment or borderline personality disorders are more likely to experience unrequited love or even become obsessed with a person.

In the most extreme and rare form stalking and erotomania can happen as a result of psychosis. These instances are actually disillusion. Normally this occurs with those who have a history of bipolar or schizo typical and are symptomatic, not under a doctors care or taking anti-psychotics.

The literature describes the changes which happen in a normal person's brain caused by love not as delusion but as bias. It has been shown that if you trust someone and you love them your more likely to overlook their faults and systematically misremember their transgressions. Moreover though beauty is in the eye of the beholder it is likewise enhanced by love. The person you love appears more attractive than they would otherwise. This is probably an evolutionary adaption to promote long lasting relationships.