Ten reasons, with mechanisms.

1. Dopamine reward. New romantic interest triggers reward circuits similar to mild addiction.
2. Norepinephrine surge. The "butterflies" feeling — alertness, focus on the person.
3. Attachment systems activate. Oxytocin bonds you beyond initial attraction.
4. Self-expansion. New partner expands your sense of self (Aron's research).
5. Familiarity + novelty balance. Similar enough to feel safe; different enough to fascinate.
6. Synchrony. Physical and emotional attunement produces felt closeness.
7. Shared laughter. Lowers social barriers faster than any other mechanism.
8. Vulnerability reciprocation. Disclosure begets disclosure; love grows in the revealed spaces.
9. Physical touch. Oxytocin and bonding.
10. Proximity over time. Mere-exposure effect combined with deeper engagement.