An erection is a very complex reflexive process that depends on the neurological, psychological, hormonal and vascular state of the body at the moment, and is influenced by our consciousness.
When there is no sexual arousal, no erection, the so-called cavernous bodies (corpora cavernosa) responsible for the erection of the penis are in a relaxed, contracted state.
Source: http://somossexualesconcientes.blogspot.hu/p/el-sistema-reproductor-masculino-el.html, http://dreampenisguide.com/blog/wp-content/uploads/2014/10/diagram.png
To get an erection, you first need stimuli (different visual, olfactory, tactile stimuli) that trigger a neuro-biochemical reaction in the brain. During this reaction, a stimulus-transmitting molecule, nitric oxide (NO), is released, which causes the smooth muscle cells that make up the cavernous body of the penis to relax via the parasympathetic nervous system. The relaxation allows much more blood to flow into the penis, causing the cavernous bodies of the penis to enlarge and the veins responsible for the blood flow to be pressed against the outer wall of the penis. The inflow of blood increases and the outflow through the veins decreases, making the penis bigger and firmer, thus creating an erection.
Source : http://i.kinja-img.com/gawker-media/image/upload/t_original/19d5bsdvffcvtpng.png
For an erection to occur, it is therefore important to have a sufficiently stimulating sexual stimulus, as many nitric oxide transmitters as possible, and an intact nervous and vascular system.
From the above, it is easy to understand how important it is to ensure that the tissues responsible for erection in the penis are sufficiently supplied with Nitric Oxide.
Read about the possibility of increasing tissue nitric oxide levels here: