Complex sentence: Caroline forms a strong bond with Martin; however he is much more enthusiastic about their relationship than Caroline is and rapidly starts to fall in love with her, especially after Caroline somewhat inadvertently kisses him at the housewarming party.
Split into simpler sentences: Caroline forms a strong relationship with Martin; however he is much more enthusiastic about their relationship than Caroline is. He starts to fall in love with Caroline ever since she kissed him at the housewarming party.