Researchers in Italy discovered that when you fall in love, your hormones go haywire. For example, your stress hormone cortisol skyrockets. That's the reason many people find it hard to eat or sleep during this time. Also, your happiness hormone actually decreases in activity. This is counterintuitive; most people indicate that during this time that they feel deliriously happy. But the reason you feel happy is because part of your brain, the amygdala, actually deactivates—that's the part of your brain that would otherwise be sounding the alarm because your stress hormone is so high.
So even though your anxiety is high and your happiness is low, it doesn't feel that way because the part of the brain that should be telling you that has taken a vacation. And, if that's not bad enough, London researchers also found that your ventromedial prefrontal cortex has deactivated during this time as well. That's the part of the brain that judges yourself and the other person. (Now you understand the saying, "love is blind.") During this time, you can't really see your lover for who they truly are. But, since you're not judging yourself either, you are quite happy and content with your circumstances.
Of course, for anyone who has fallen in love, you already know that this is a temporary phase. Eventually, your brain must return to homeostasis, or relative stability. When this occurs, some couples break up, and others move to the next phase.