Samantha is a dreamer. Tim has chronic allergies and frequent nosebleeds. It was a combination of these which brought the star-crossed lovers together.
Samantha was standing in a crowded subway car. As she drifted off into a delightful daydream about marshmallows and baby ducklings, she missed her stop.
Tim, returning from a doctor's appointment, stepped into the subway car and took a place next to Samantha. He then pulled out his new-prescribed inhaler (meant to combat his chronic allergies), and as the powers that be would have it, he had an allergic reaction to it. He coughed, sneezed, and swelled up like a tick about to pop. The commotion snapped Samantha out of her daydream, and quickly realized that she had missed her stop. In a fit of rage, she dashed out of the subway at the next stop and took a cab to work, where she was fired due to her constant tardiness.
Then they got married.