Our boners are a result of relaxation. I know it seems counter-intuitive that something can be hard as a result of being… well… soft, but the muscles/tissue in the erect penis are actually relaxed, not contracted, and the flaccid penis is actually contracted. Think of it this way: when the tissue is tense, blood can’t fill it up and make it hard, but when the tissue is relaxed the blood flows in and it swells up.
This is why we are known to get boners during massages and other relaxing activities.
This is what causes morning wood (not sexy dreams).
This is why infants who can’t even conceive of “sex”, let alone get aroused, can get boners.
This is one of the reasons we get unwanted boners.
This is why we sometimes can’t get it up (or just partially), even when we’re really into you.
This is also why the more pressure we feel to perform, the less likely a lot of us will be to get as hard as we’re able (in those of us without a medical condition, that is). Obviously it has a lot to do with how a guy responds to stress, but for a lot of us, the more excited we are, the harder (pfft!) it will be to get hard.
“The penis is a contradiction.” - Stephen Hawking