Why is it painful to have an erection that lasts for a long time?

3 hours: An erection is essentially trapped blood in a confined space. If the blood is trapped too long, it can no longer provide oxygen to the penile tissue and results in ischemia, strangulation of tissue. This is painful. Typically, an erection lasting longer than 4 hours is considered critical for permanent damage to the penis.

Congestion: trapped blood paw can cause the pain