When Haruhi was in sixth grade, she went to see a baseball game with her parents and was shocked by the sheer amount of people attending. After finding out that the large amount was only a small piece of the entire world, and realized that she was only a small part of it, Haruhi believed herself to be ordinary, and deduced that if there is someone out there with an interesting life that it should be her. This results in Haruhi's desire to find strange things.
During elementary school, she came to know Sasaki, under the surname Sasaki was using at the time.
In middle school, East Junior High, Haruhi becomes known for several things, such as putting talismans up around school, dating and often immediately dumping every guy who asks her out, and, most notably, drawing a huge graffiti mural on the field on the night of Tanabata, year 0. Kyon was actually the one who drew this under Haruhi's orders after he and Mikuru traveled back in time. (According to Yuki Nagato, the mural meant "I am right here".) Thanks to the dark, Haruhi could not make out his face, and so Kyon used the alias "John Smith" to further keep himself secret. "John Smith" implied to Haruhi that there were strange things out there and that there might be interesting people at North High, inspiring her to attend the school.
Shortly after, the same night (from Haruhi's perspective), Kyon shouted out to her "Cheers to saving the world by overloading it with fun with John Smith!"
The consequences of her ability erupting was a timequake preventing time traveling to before this point. This also seems to have sundered the timelines of Fujiwara and Mikuru Asahina.
Until she went to high school, Haruhi sought "John Smith" at other schools, but was unable to find him.