Body Language: Why do we act differently around someone we're attracted to? Many professionals have said that when you're attracted to someone, you tend to fidget, run your hands through your hair, sit with your legs facing them, and that our faces light up around them, which are all actions that happen involuntarily. Why does this happen?
Generally speaking, romantic love can be considered an emotional process. From an embodied perspective, emotions are to a large extent interactions with sensorimotor processes (connected to the body). http://www.iep.utm.edu/emotion/ provides a good overview of the various emotional theories, of which the feedback theories are my favorite. The feedback theories essentially state that body states cause valenced (positive or negative) feelings in a bottom-up manner, and that cognitions (higher-order thought on a 'symbolic' level, such as appreciating someone's qualities) cause embodied reactions in a top-down manner (these are the quirky love-induced reactions you speak of). This goes on in a feedback loop. In this process, neurotransmitter systems are highly important. One of the most popular theorists out there in this area is neuroscientist Antonio Damasio - his somatic marker hypothesis is a really interesting theory: http://www.iee.unibe.ch/unibe/philnat/biology/zoologie/content/e7493/e7854/e8920/e8926/DamasioPhilTransRSocB96.pdf. (He applies it to decision making in the end, but the basic principles are all there) If you're interested, you could also read his book "The Feeling of What Happens".

I would say that these quirky little habits we have when we are nervous around someone we care about in that special way are mostly unintended side effects of a significant release of neurotransmitters (like dopamine, serotonin, etc.). These neurotransmitter systems are connected to several brain areas, but especially also to our action systems, causing us to fidget, smile, shiver, or otherwise display exceptional behavior. Of course, some of these behaviors are socially adaptive - smiling is useful to communicate that you appreciate the other person, for instance, whereas fidgeting is probably not socially useful. The reason there is such a strong release of these is that romantic attraction is a mixture of a emotions/desires which are very important to reproduction. For a light (not too academic) read about the biological basis of love, I recommend http://www.amazon.de/Why-We-Love-Chemistry-Romantic/dp/0805077960. For a condensed version in a TED talk, see http://www.ted.com/talks/helen_fisher_studies_the_brain_in_love.