Why do guys get boners? Let’s start with the basic. A boner or erection occurs when there is an increased blood flow in the penis, the blood will travel through a series of veins and arteries, causing it to grow and harden. Essentially, a boner is a blood filled rod. While it is mainly caused by sexual arousal both physically and mentally, there could be other reasons, like puberty and morning wood (or the gender equivalent of women getting “wet”). Often times, the size or quality of the erection can be affected by both mental and physical health, something like an anxiety could entirely stop a guy from getting hard at all. So a guy gets a boner when he is sexually aroused and so your brain went full throttle and pump your penis with blood.