why didnt the dementors effect harry than other people?

Lupin explains in the third book/movie.

Lupin: The dementors affect you more than others because there are true horrors in your past, horrors your classmates can scarcely imagine. You are not weak, Harry.

Because he had been given chocolate.

I think you have the question backwards. The dementors affect Harry more because he has suffered more than the others. Dementors make their victims relive their worst memories. For Harry, this was seeing Voldemort murder Lily right in front of him. It would be enough for anyoned to faint.

I FIND it odd that the dementors had no greater effect on Neville, given the horrible things that have happened in his past.