There's a reason we speak of "good chemistry" when we refer to our connections with romantic partners. Various hormones, chemicals, and neurotransmitters are responsible for our growing romantic/sexual feelings. As these hormone levels increase in the body, you begin to realize you're physically attracted to the other person (with exceptions such as people on the asexual spectrum). And, of course, desire or lust can happen independent of attachment and without leading to love.
Once we become attracted to someone, the reward centers in our brains begin to fire and release dopamine, norepinephrine, and serotonin. Simply being physically close to them causes a release of dopamine, which makes us feel rewarded.
The feelings of giddiness and euphoria are thanks to norepinephrine. When your heart races and your palms sweat, you can thank norepinephrine; it's even responsible for the sleepless nights and the lack of appetite we tend to experience when first falling in love.