Esposito had been a well known playboy before he started dating Lanie, and thus he would often be asked what it was about her that made him change his ways and want to settle down. When presented with such questions Esposito would merely shrug, smile and subtly change the topic, however many people had come up with their own answers.
Montgomery had always believed it was because of Lanie's ability to put Esposito in his place, instantly earning his respect. Their banter was legendary long before Castle arrived on the scene and Lanie generally got the last word, a feat very few people could admit to.
Ryan, knowing what Esposito often saw in women, believed it was her arse – which earned him a smack from not only Esposito, but Jenny too the one time he was brave enough to mention it.
Beckett believed it was Lanie's wit. She knew from experience just how quick Lanie was with words and had observed Lanie's and Esposito's word spars with awe and wonder. There were very few people who could keep up with Lanie, Espo was probably the only person she knew of.
Kapowski, after catching them making out in an investigation room, swore that it was her kissing that won him over. When recounting her tale she would always blush and admit that just looking at them had enflamed something within her too. The idea that the two were so passionate was certainly no foreign concept to all who knew them.
Castle would smirk and blame it on the ME's dress sense. Some of the outfits she wore were enough to blind the Pope, and Esposito was indisputably very fond of the female form. His comments had earned him a smack from Beckett and Lanie, who had been walking past and overheard his comment.
Alexis, who the adults deemed too naive to understand, attested it to Lanie's smile. She's spotted the softening in Espo's eyes whenever Lanie smiled and couldn't help but believe that was why he loved her.
And all of these things were reasons why Esposito loved Lanie, some he'd admit to her if asked and others he'd keep to himself... an possibly Ryan cause man that arse was too good not to brag about occasionally. But none of them were the reason he FELL in love with her, and yes it definitely counted as falling. Esposito has absolutely no plans for settling down for at least another 5 years, probably nearer to 10, but fell in love he had. And as to why?
Javier Esposito, if you caught him in a particularly honest mood, might tell you about how it all started on an average Wednesday. How he'd just popped down to the lab to follow up some average lab reports when he came across an un-average sight. Instead of working, Lanie had gotten caught up in the radio and was dancing around the morgue in her scrubs. The sight almost hypnotised Esposito, and he couldn't help but stare at the woman in front of him as she swung her hips completely uninhibited, to the rhythm of the music. He'd seen women dance in clubs before, but he had mostly been focused on their clothing (or lack of it), rather than their dance ability. However the woman before him was fully covered in scrubs, and yet he found her far more fascinating than any woman he had ever seen.
When she turned and saw him, he was expecting her to act shy, stutter a bit and pretend it never happened. Instead (in what he'd now term 'true Lanie manner') she simply winked at him with a cheeky smile on her face and continued dancing until the song was over. And that, dear reader, was the moment Esposito knew that he was in love with Doctor Lanie Parish. He had asked her to come dancing with him that evening. She had insinuated that he was merely going to take her to a club and use it as an excuse to gyrate against her all evening. When he had instead offered to take her to a latin club for a night of salsa and samba dancing, she had been pleasantly surprised and ended up agreeing. And the rest, as they say, is history.
In all honesty you are more likely to get the story from Lanie.
The only other person to have a clue was Perlmutter who had walked in on the couple dancing around the morgue on a few occasions and could clearly spot the love written all over Esposito's face during those moments. Although that being said, Perlmutter was one of the few people who couldn't care less about Esposito's reasons for loving Lanie so there's no guarantee he picked up on it at all.