The idea that we only love our family according to biology is not true, but its also not clear what people mean by the word 'love'. There are many ways to interpret that word!
Hope this doesn't totally suck any romantic ideas out of you, but metaphysical concepts of love and romantic ideas of love are not always relevant when you talk about biology. A case for the universal human family and world peace can be made.
In evolutionary biology love usually has a different meaning than most people mean, but maybe one similar to what you refer to here; Love is often considered to be the bond with one's offspring to help ensure that they succeed and reproduce.
This idea is most often expressed in terms of "kin selection". A theory that an individual is helpful to your reproductive success if they carry genes similar to you. In particular ant colonies may be entirely built around the solidarity of the family unit: all the workers are birthed from the queen and they themselves do not reproduce, but they do work together because, to put it plainly, helping the hive is helping themselves reproduce.
The biologist Sarah Hrdy has written an excellent book that looks at the biological origins and variety of ways that motherhood creates social and emotional relationships, which is not entirely a human idea of motherhood, but biological and mind expanding for the evolutionary take.
However, the idea that only close relatives should be loved is not quite right. Even intuitively we see adults adopting children distinctly unlike them in appearance, people devoting their lives to the general good, etc etc. Biology cannot pretend that this sort of behavior doesn't exist - it can only try to explain it within the framework of what we understand.
Its well understood that cooperation and social cohesion are also the result of evolutionary forces. The primary reference for this is the iterated Prisoner's Dilemma experiment; in systems where individuals are interacting over and over again, helping others creates a system of cooperation where reciprocity where everyone benefits because the payoffs are stability, security, safety etc.
Again this might not sound like 'love', but in human social, cooperative situations it can be professed as love. Love of country, profession, causes etc if you will.
There is a big debate about whether altruism exists: many biologists believe that an extended from of kin selection is in play - that we're all one sort of human family. This is essentially an argument that all our energies are self interested on some level - we're just trying to further the interests of the human race because it will help our own offspring do well.
I think it might go further than that - we also care for non human and even ugly very distant forms of life if you look at some people's lives. One can argue that the realization that caring about the ecosystem is self-interest and not altruistic. The distance at which this extends kin selection is pretty far, but anyway the argument goes on.
Additional Points: Lots of human interests like science, are a sort of love? They don't seem to help reproduction.
This is a pretty deep point, really. I'd say it can help to understand question to start with uncles and grandmothers. Lots of animals do not reproduce, and this is considered okay by kin selection. Take a look at wolf packs. The alpha male is the only animal to mate, the other males seem to be hanging out. First of all the wolves may be closely related, so the avuncular role of taking care of cousins and nephew/nieces is one kin selection argument. But even if they weren't, there is the fact that pack structure is not biologically altruistic; the other males and females who do not mate may still do so in the future. In the meantime, the pack cooperation creates a more secure condition where offspring can have a better chance of success. Lots of creative, cultural, scientific work is similar as far as I can see. We can adopt non-reproductive pursuits and still feel that we are doing something worthwhile. I guess the same can be said for adoption, social causes, religion and just about anything else people do which is not having children.
How can this be a selective advantage - included by evolution into the human instinct? I'd cite the grandmother effect here. There is some evidence to support the idea that women live longer after menopause because it creates more successful grandchildren. While this is not a direct effect, any mutation that extends the caregiving instinct would be sustained in the gene pool. Its possible that if such a caregiving instinct of uncles and grandmothers were to extend to other people, even beyond the family, that it would be sustained in the gene pool.
Indeed being smart and or rich usually doesn't increase the number of children humans have. In fact the opposite is the case for both statistical and anecdotal evidence. Few would say that this is a bad thing. Human being are getting a better deal when they spend energy on better quality offspring, not quantity. You can look at the r/K selection theory for a better understanding of this argument, but some animals have chosen cheap offspring and many of them - think cockroach or guppy. These animals would rather eat their offspring than try to feed it. Humans, more than nearly any other animal i believe, spend their lives caring for helpless offspring until they are ready for life. Modern life has only extended this from a few years to nearly 20 years. But it certainly works - the only animal that can compete with human beings is other human beings.
Hope that makes sense. There's some speculating here, but its hard to dispute that people devote their time and energy to all sorts of things besides mating. In the end biology has to accommodate such behavior. Biological theories we call laws don't change the behavior of living things; its always the other way around.
