"intense affection", "a feeling of attraction resulting from sexual desire","enthusiasm or fondness", "a beloved person", "to enjoy enthusiastically". Scientists can't really define love any more than that. I've read some things related to the increase of a person's blood pressure when they see a "beloved" person, but that's about it.

I find it kind of nice that science can't explain everything, especially this topic, how boring the world would be if everything was easily explained.

I think that the interesting part of this question is that it asks for a rational description of irrational behavior.I am not saying that this is not possible, but that it is outside of the realm of "pure" science. Social scientists or psychologists are better fit to reply than ecologists, biologists, chemists, etc. I would venture to say that love is a term to describe a set of behaviors but that is as far as I can go as a scientist. On a more personal level, I will also say that some behaviors such as spouse or child abuse have been justified by perpetrators as expressions of love. So has emotional abuse. Therefore, a rational definition of love should also include the effects of the behaviors on the objects of affection. You can see how a rational definition gets into trouble here in defining love.

I am not an expert on the science of emotions, you could say that I'm just interested on the topic of emotions and that I read about them on my spare time, so take my answer merely as an educated opinion rather than an expert one.
The science of emotions is indeed a difficult one. Most work done in this area belongs to biochemists, neurologists and psychologists. Much of it tries to relate the effect of release of certain chemicals in our bodies (mainly hormones) to the brain mechanisms that they produce or inhibit. There is also a lot of work done on how evolution has produced certain connections inside our brains that respond in particular ways to certain stimuli, thus creating behaviors that may be included in the area of emotions. We can give two examples:
The love of parents towards their kids can be atributed (among other things) to the instinct of survival of the species; the young ones of any species are the future: if they die, the species will become extinct. Therefore, species where the kids are better protected tend to be more successful in staying alive. The love between a man and a woman can also be atributed to (among other things) the same need of survival.
The feeling of "being in love" produces in our body the release of certain chemicals that soothe the brain and create a state of well-being that is very pleasant. Some scientists claim that the ingestion of chocolate produces a very similar response; maybe that's why people like chocolate so much. ;)

In the first example, we have a biological origin for love, but it doesn't explain several things, such as: Why do we love some people more than others? Why is love sometimes so selfless? What about the brotherly love inside a family or a group of friends? The second example tells us how it's possible to explain that love is a pleasant experience per se, without waiting for other rewards, but it doesn't tell us what triggers it, what makes us love.
For biology of emotions in general, the first chapters of Daniel Coleman's "Emotional Intelligence" can be an interesting further reading, depending on the age of the students.

According to Eric Fromm, love can arise from the need of the human being to be connected to others; when we take conscience of ourselves, when we realize that we exist and we're different from each other, we can also feel small in comparison to the big world in front of us, and that makes us want to be connected to other people in order not to feel so small. However, that author also maintains that true love (of every kind: love between parents and children, between friends, between a man and a woman) can only exist if it comes together with a certain choice and commitment. Otherwise, it's just an emotion that can come and go with the wind. This last paragraph is more related to the psicology of love rather than the biology of it. Depending on the age and maturity of the students, some passages of Eric Fromm's "The art of loving", selected by an educator, could be interesting. This approach relates more the origin and mechanisms of love to psichology.

Last, but not least, people have also looked for the origin of love in religion. Most religions believe in the general principle of "love thy neighbor". They also link the ability of loving (and hating) to the ability of making free choices: the human being is created with the ability to reason, to choose between good (love) and evil (hate), and this makes us different to the animals, in the sense that we don't only have emotions (that could be explained from a purely biochemical and evolutive point of view) but also feelings (where psichology has had a lot of work).

So, after all this, what's it going to be? Well, the answer is not clear, but my personal opinion is that what we, very loosely, call love, is a mixture of all of the above. There are emotions that we associate to love (when we meet for the first time this boy or girl that makes our heart beat faster, when we say "look at that little baby, isn't he/she cute?", and so on), there are choices and commitments that we associate to love (the relationship inside a family where everybody sticks together despite ups or downs, forgiveness of friends, and so on), and there are scientific explanations of all sorts to them, provided by different fields in science, from pure biochemistry to psichology. But I believe that the origin of love is deeper than that. I belive that the ability to love (not only as an emotion, but as a deep feeling that can guide our life) is one of the things that truly separate mankind from the rest of the animals, and it would be a shame not to use it for good. Finally, I do believe in God, and I believe that he has put in us this ability to love (though of course this is not scientific at all...), but if I didn't believe in God, probably love itself would be

I'm not going to attempt to answer this question since it is not my field.
I do however remember taking a class in psychology many years ago and that this was a topic that was discussed in some depth. So, if you don't get a satisfactory answer, I suggest finding a basic psychology textbook. There should be information in there for you.