op you gotta make up your mind, are you goin at evolutionists or atheists? they're not one in the same...
anyways what does love have to do with evolution?

is love necessary for procreation? have you ever heard of a "love" gene? i don't think there's anything necessarily 'mysterious' in the question why animals don't fall in love the same way we do, animals lack a lot of the abstract concepts and emotions we have, simply cuz we have a higher level of thinkin. i could just as easily ask you to explain why animals don't show envy or greed, etc.
this is only my interpretation, but if you're tryna get a semi 'scientific' answer i think you'd have to look at maslow's hierarchy of needs. we all have the physiological urge to procreate and thats pretty much where the evolution part of it ends, but the rest of the needs, safety, psychological, self esteem, etc would be where love develops... your experience when partnered with someone that meets your personal needs