system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How does sexual arousal occur?

How does sexual arousal actually occur? What causes it? Why do human beings want to have sex?
gpt
Human beings want to have sex because we're animals and animals exist to reproduce, it's hard-wired into our evolution because any animals that mutate the desire not to reproduce will... Well... Not reproduce, so they will die without passing on that trait. 

Sexual arousal likely exists because keeping an animal in a constant state of being prepared for sex would be a waste of resources. It's more efficient to have those aspects of our biology only trigger when we think we're about to enter into a situation where reproduction might be possible, when we haven't reproduced for some time or when we encounter a partner with desirable genetic traits. (ie, when we witness erotic imagery or situations, when we haven't had sex or masturbated for a day or more or when we see a person we think is attractive).

The result of the first of those situations (recognising that reproduction might be on the table) is that it ends up being the brain's job to decide when that might be. That means there's a mental component to sexual arousal, it becomes a vague (and scientifically difficult to study) field wherein societal and personal factors begin to influence our sub-conscious recognition of what is or is not sexually arousing.

Our ancestors frequently carved imagery of breasts and swollen pregnant bellies, images of fertility, which have a sensible enough correlation to reproduction; but today we have many other (often less relevant) mental associations (think of people becoming aroused at the sight of underwear or bikinis, even with no one inside them), so the brain's involvement with sexuality is definitely apparent, but difficult to understand.