Arnold was a little too naive and trusting for his own good.
Growing up, I've always loved and admired Arnold's good nature.
However, I now realize that he's extremely naive to the point where he allows people to take advantage of him and treat him like crap. Most of his " so called" friends were pretty fake and left him out to dry.
1) Sid, Stinky, and Harold:
In the Full Moon episode they made Arnold take the fall b/c they were scared of getting in trouble.
2)Sid: In the bag of money episode he accused Arnold of being a theif and Arnold still decides to forgive him at the end.
3) Stinky and Sid: Spilling Iggy's secret after promising they wouldn't.
4) Iggy:
Forcing him to humiliate himself by wearing a bunny suit just to earn his forgiveness, and Arnold being desperate enough to make piece with him decides to go along with it.
5) Gerald:
Almost didn't believe him when Sid accused him of stealing money.