And he tends to default to sarcasm and yelling (and sarcastic yelling) when he's with someone he has no patience for, and that would just freak her out even more, creating a positive feedback loop of Fluttershy crying and Lassiter yelling.

(On the other hoof, if they were going for cookies, she would be his best friend in seven seconds flat. Especially since he supposedly does have a soft spot for animals- except for squirrels- and that would endear her to him.)