TimmyG hit the nail on the head.
Smells trigger those strong memories due to the proximity of the memory center.
As far as species besides our own (and actually those species are bonobos and dolphins) that's not exaaaaaactly true. It depends on what you mean by "pleasure". The only thing that can be said conclusively is that they have sex at arbitrary times or regularly. Monogamy, polygamy, homosexuality, and masturbation exists within nature as well, but obviously it's in a different way then it exists within human beings. We've built cultures and businesses around our sexuality.
I still maintain that reptiles absolutely do not have capability for emotion. It's just my opinion, but I think it's pretty naive to think they do. Reptiles were designed to respond purelyto their sharp instincts, and there is absolutely nothing wrong with that.