Erotic love (as opposed to parental love, for example) is intimately bound up with male parental investment. Why?
Species with high male parental investment use a reproductive “strategy” in which bringing offspring to viable adulthood normally depends on support from both parents. Why “normally”? – A single parent might get lucky and manage it in times of abundance, but even then, the resulting adult will have to compete against other adults who have enjoyed the attentions of both parents. That will usually be a disadvantage. We know it must usually be a disadvantage, because if it were not, the alternative one-parent strategy would spread throughout the population and become the norm. A male who is absent for the rearing of offspring can wander off and father other offspring. A female who does not need a male partner to rear offspring can choose from a wider variety of males, some of which will be of higher quality than others. This is what does happen with many animals such as grazing ruminants, and it happens because – given the specific needs of their young – that is a more efficient way of producing viable adults.
Please note that although different species use different reproductive strategies, it is wrong to suppose that there are smooth gradations between them. The most successful strategy will always spread throughout the population and become established as the norm. Some species simply lay eggs and leave the young to fend for themselves. In other species, mothers play a special role as parent. In monogamous species, both parents play a role like the role of mothers. Their parental investment will be roughly equal, because their “biological interest” in reproducing is the same. Which strategy becomes established as the norm is determined by which is the most efficient method of producing viable adults in the next generation. But there can be little or no crossing over between these different strategies.
In species whose males do not wander off, sticking around to help provision the offspring isn’t just an added luxury for the female. It’s a matter of life and death for the offspring, and thus a matter of reproductive success or failure for both male and female. Since proliferation of genes in future generations is evolution’s “prime directive”, it’s a matter whose importance equals that of life and death for everyone involved. The male isn’t simply doing the female a favor – he’s using her to reproduce, just as she’s using him to reproduce.
When provisioning is a matter of life and death like that, a male who misspends his provisioning powers on another male’s offspring is in effect throwing away his ability to reproduce. And a female whose male squanders his provisioning powers on another female’s offspring in effect has her ability to reproduce stolen. Given evolution’s “prime directive”, these possibilities are bad news for one or other of them. Furthermore, the strategy of sharing provisioning between male and female opens up such possibilities. Monogamy and betrayal are two sides of the same biological coin.
So in species where male parental investment is high, something new enters the picture: potential parents of each sex set “terms and conditions” for each other in a partnership whose “purpose” is to bring offspring to viable adulthood. Each demands guarantees of fidelity from their partner (at the same time as being rather more relaxed about their own fidelity).
Whatever we choose to call this set of attachments and demands, it is close to the everyday folk psychological concept of love. It isn’t a selfless or sexless ideal, or an experienced “feeling”, but a real attachment between two members of a pair which serves a vital biological function. It involves possessiveness, jealousy, and the ever-present possibility of betrayal. In real life there are many variations on the theme of two parents exclusively attached to each other, of course, but I would argue that most of them involve some degree of betrayal of one sex by the other, even if those involved bite the bullet and observe the social decorum of calling it something more polite.
You might think this is a rather bleak view of love that “lowers humans to the level of animals”. But I would urge you instead to think of it as raising some animals (such as birds) to the level of humans.