Two days ago when I held an 11 month old child. It's called 'infatuation' and it is a neurochemical effect that can happen at any age and not just in romantic situations. It lasts about 3-4 months (when consistently exposed to the person). When we are exposed to the person that gives us 'butterflies', or a pleasant, uplifting feeling, it's because a number of chemicals in our brain are activated, and produce 'endorphines', which make you feel verrrryyyy niiiiice.
Unfortunately, infatuation, as we know, is not love. But also unfortunately, most people who are romantically infatuated do not wait for the 3-4 months to wear off to see if it is really love, and have sex. And of course, sex produces all kinds of BONDING brain chemicals to be released. All in all, ignorance creates a lot of relationship pain (and illegitimate children).