Nature wants all species to replicate themselves, so it's built in. I would ask another question, 'Are humans the only creature to actually enjoy the mating act?'
Why is it that humans are able to reproduce before they are fully mature and able to care for their young?
This is something that can only be surmised. But it can leave you with other related questions as well. A female lion can mate up to 100 times a day with multiple partners when she is in season.
Now, that's a girl who gets around (Not judging here), but if she was doing it for the pleasure, then why wouldn't she be doing it all the other months of the year. Yes, sure, she would get a reputation, but girls just wanna have fun, right?
On, the other hand, anyone who has ever had a dog hump their leg surely realizes he isn't doing it to make puppies, so he's gotta be gettin something out of it. I don't even want to know what his response is to the other mutts who ask him how his date went with his owner.
On the surface, it would seem that hooking up would be a very strange thing to seek if it didn’t bring some form of pleasure. It increases risk of disease, it wastes energy, it can seriously increase the likelihood of something bigger coming along and eating you, and it often leaves you with responsibility.
I don't think it's all about reproduction, as some animals get busy even when the female is not fertile. I think for some, it can be an intimate way to cement a bonding. However, with others, they don't even bother to call the next day.
To use a worn-out phrase .... it's complicated.